Your gonna get alot of people saying don't do it. I actually agree if you're asking those kinda questions. I do it, but I have 14+ years experience mainlining drugs. I sometimes get asthma attacks after smoking, don't have any maoi's for oral, don't like snorting, and refuse to IM (it hurts, and can cause absesses).
So anyway, to me, it feels alot cleaner. I always refused to shoot dmt at first because I'm a recovering H addict and the whole ritual I thought would bring bad vibes into the trip. But thinking about it while in hyperspace, I feel I was told to go this route and I would be rewarded.
Because of my experience I have a technique. First!!! Make sure you have super clean DMT fumarate. My IV batch is perfect. Re x'ed several times ect. ect. Always use a new clean needle! They are free at needle exchanges or super cheap at pharmacys. That's why I'm still alive, don't have any diseases, or track mark/collapsed veins after all my years of abuse.
I weigh my dose and consider you need slightly more with fumarate (don't even know the percentage) but I did 40 mgs. last time. I use distilled h2o, boil it, and use the least amount of water I can to make for a quick shot. I put a bandage halfway on with an alchohol swab on it, and inject it as far from my brain as I can (my hand). I push it in quick, put the needle in a preplanned container next to me, slap the bandage on all in about 3 seconds. Then I have 7 seconds approximately to lay down or whatever.
I have to admit, smoking it is a little more intense. And the duration is the same.
I am going to repeat that I'm not recommending this, just answering a question for educational purposes. I am very responsible with IV use. I had a babysitter and a small dose on my first experiment, but don't like people in the room with me when I voyage. So I had to be able to do it myself.
I'd just suggest stick to smoking, or oral. Needles are usually the best way to administer drugs if done correctly, but it's just a bad scene to get into if you've never done it.
I have an overactive imagination, and am certifiably insane. So anything I post is purely fictional.