A dose is about 3-7g mimosa, but better for first time take 3g or so. You can boil more doses at a time and just divide the final liquid in proportional ammount to the doses.
So anyways, put mimosa in a pot, add some water to cover the mimosa. No need to measure, but if I had to guesstimate I would say around 50-250ml water per dose per boil. Have the pot covered for water not to evap, and put it on low fire. So after 1 hour (no need for so many hours) you can pour that off leaving the bulk mimosa behind (you can pour through a very coarse filter like a metal strainer or an old t-shirt, or use the top of the pot to help you hold most mimosa on the pot). Add fresh water and boil again for another hour. Do this for a total of 3x, putting all the liquid together and discarding the solids.
Then filter the liquid once again with a bit finer filter (like a loose cotton pad/plug in a funnel, for example), to get rid of more inactive solid material. Then you can reduce that to small amount by simmering (try not to boil or very light boil only). I would say that each dose should be about 80-100ml of liquid.
You're gonna need to take some MAOI together/before this, I guess you know. So its basically doing the same 3x boils but with 50g ayahuasca vine or 3g syrian rue