I made some test flights yesterday, I'd appreciate your thoughts on some questions I have.

Machine
I was shopping for the right glass and it seems that I found the perfect one. It's 100ml, similar to this one, but with longer neck. I've drilled 6mm hole on the bottom and put a paper straw through it. I take a steel wool, put a spice inside, squeeze it together tightly and then put it up the bootleneck. It feels like loading a gun.

First bullet
~5mg, but I feel my scale is not very precise. Nervous and excited. Fired up my torch, slowly inhaling. I felt a very subtle taste of the spice. Exhaled almost immediately. Waiting for a hit, but nothing happened. Absolutely nothing.

Then I recalled I did it all wrong. I should've emptied my lungs and hold the breath for as long as I could.

Second attempt, I exhaled, lighted up the torch and hold my breath this time.

Very gentle visuals, a wire from a phone charger started to vibrate slightly. I felt it worked, although very gently. I wanted more.

Second bullet
I took out the steel wool and investigated the inside of it. The spice was gone, although there might've been some leftovers. I put 7-10mg more.

I took a hit, visuals were stronger. I looked at my bed and I saw blue dust particles coming off it. Something like, if you put a very fine dust on the bed cover and then slapped it with your hand, there would be small dust particles in the air. I saw faces in these particles. For a very brief moment I felt bit scared that this is just a beginning and it'd get crazier, but I thought "it's ok, nothing wrong is gonna happen" and it calmed me down immediately. I felt in control and I was smiling. Quickly visuals faded off and I was in a state where I didn't "sober up" completely but the visuals were almost entirely gone. The closest I can compare it to is when you just woke up from a dream and still have some images of it. I wanted more.

There was a pattern:
1. Taking a hit (sometimes two)
2. Trip
3. Effect wearing off, almost "sober" but not quite, thinking "I want more" at going back to 1.

I don't know how many times this pattern occured, 6-10 times I'm guessing.

During second or third hit I put the fire closer to the machine and the smoke filled it all immediately. It was like an explosion. I though it will be a very good it, so I focused, inhaled and held it. As I was holding it the trip started and I realized I haven't exhaled in quite some time, but I wasn't worried, I had total control over my breath. I was deciding when to inhale and when to excale.

In the end I had a couple of trips. Here's my attempt to organize the most interesting events.

One time I looked at my hand. Between palm lines and veins there was a beautiful woman dancing slowly. Then I looked around the room and saw two almost transparent beings, one hindu-like and one like chinese dragon. They felt more like a watermark on a picture. In the corner I saw a small beam of light. I thought that if I took more I'd be sent through that light and I'd see what I've read about in other trip reports. It seemed like an invitation.

Sometimes I felt that I'm switching between two modes, one that wants to go deeper and enjoy and the one that thinks things like "objectively, you must look dumb as fuck, with a bottle in your hand, smiling, gazing at the bed cover", which would sober me up.

By far the most interesting part was the "realization" about words, which might've been part of very gentle ego death. At certain point I was able to look at words with a fresh perspective. They appeared to me as blocks, little packets of meaning, which dominate my life. I don't remember the details, but it was around the idea that words are more than mere tools, they're what the mind is made of. For example, words are not something we use to express emotions, it's that emotions and words are the same. Now I feel disappointed, because this idea doesn't make as much sense as then. I said "words" in my native language out loud a couple of times.

Part of the "very gentle ego death" is that I felt that boundaries between me and environment were a bit blurry. For a brief moment when I started to sober up I couldn't recall where am I geographically. I tried to iterate over possible locations, but the concept of "location" became liquid and wasn't useful. Something similar happened with time. It wasn't scrary, though, I enjoyed that part a lot, at least partially because I knew I was already in the process of sobering.

In total I had 6-10 trips during 45-80 minutes.

Third bullet
I chilled for an hour, took a shower and decided to take last hit before sleeping. I was a bit tired and sleepy.

I took the steel wool and eyeballed some spice, not too much, around 5-7mg? I'm pretty sure there was residue from before.

It hit me quite hard, in comparison to the previous attempts. For the first time I heard loud ringing noise. Not good, I thought. Visuals were quite strong, books on the shelf started to wave. I closed my eyes and visuals were quite interesting, but not overwhelming. Colors becase warmer and room looked much more beautiful than usual.

There were two or three repetitions (as described above). After the last one, when I thought I almost sobered up I stood to put away the machine and go to sleep and then something completely unexpected happened: I was very light. I put away the machine and started running and spinning in my room, because it was so much fun. Soon I started doing so in a theatrical manner, waving hands. I saw a glimpse of my reflection in the window and mirror and thought "You must look like a complete idiot". It didn't matter, I had so much fun by moving around. I decided to take last one hit and then go to bed. When I laid down I felt a bit of nausea for the first time, probably caused by me hopping around like a ballet dancer. Had some CEV, nothing spectacular, then sobered up and tried to fall asleep.

Overall
Very pleasant and controlable experience. Eager to do more. I think it was a good test flight, all sensations vere mild and managable. I like the "flight" metaphore, so overall there were a couple of test flights at low altitude. The equipment works, technique is not quite there and there was a glimpse of what might be "up there".

Questions
It seems that my technique is off and that I couldn't vaporize everything in one go, even though I tried to inhale for a long time. I tried to hold the fire in some distance (5-8cm) below the machine, but it seemed that putting the fire almost directly at steel wool was the most efficient. Only once I got the harsh smoke, beside that it was mellow and even pleasent. Besides that I noticed a smoke coming off the bottleneck outside the bottle, I think it's because some melted spice got out.
Is there a big difference between using different bottleneck sizes and shapes? Mine is long and quite narrow.
Do you preheat the glass part of the machine?
Is there some similar substance that can be used to perfect the technique? I tried to use black tea, but didn't manage to vaporize it, but I burned it a couple of times.

I'm suprised by the fact that I was able to get a bunch of hits. It felt a bit like I was in a loop and it seemed almost addictive. Overall I'm glad it happened that way, because I'd probably sent to the outer space if I took it in one big hit, I just wonder if you have similar multi-hits sessions or is it more like you do it once and then after 10 min you're done?

I don't preheat glass part, only the mesh of the machine. Shapes/sizes don't matter that much, according to my experience.

Regarding some other similar substance - I don't know, but you are using small doses for that, which I did also.

Regarding multihit sessions - I used to have them, with spice doses on the smaller end, combined with MJ I was able to have a several "dive-ins" during 2-3 hour "session". This is my experience: After certain point, if you are really high on MJ and your mind has been already opened up to psychedelics, you might get DMT trip not like common "1) blastoff 2) omg that was beautiful, what the fuck was that", but into "waves", like with LSD/mushrooms but over a shorter period. You smoalk, then close eyes, have a 4-5minutes "dive in", open eyes then feel like still tripping, close your eyes have another "dive-in", open your eyes again, take a hit or two of MJ, feel the carrier wave again, close eyes, etc etc.

Basically small doses of spice combined with potent MJ was very recreational for me.

There is no universal receipe for happiness my friend, just follow your gut feeling (and read these forums)

Welcome!