Definitely DONT take 15 grams mimosa for the first time
indeed start with 5. SWIM's first time was with 9 and that was very strong!
If its your first time drinking some ayahuasca analogue, expect (and prepare for) purging... So have some bathroom or place in the woods to go to if necessary, and its always good to have some water/cold watermelon juice nearby...
To prepare, boil on low fire for 10 minutes, separate the liquid, put the solids with fresh water and boil again, doing this for a total of 3 times. Then throw away the solids, put the liquid together, and boil/simmer it down on low fire until there is about half a glass per dose. It will taste VERY bad.
To help a bit with the taste, one can try to do an egg-white cleaning before.. which is basically once you got the 3 filtered liquids together, but before you boil them to small amount, you get egg white, mix it good before, and then with the filtered brew on the fire, hot but not yet boiling, mix in the egg white and keep stiring. The egg white will in the next seconds/minutes become greyish and float, and together with it it will trap a lot of tannins, which are substances that give the adstringency/bad taste to mimosa. So then either spoon it out if they are solid enough, or sieve/filter it or whatever other way you preffer. Then boil it down to small amount as before.
Drinking strong ginger tea half an hour before can also help with the nausea/stomach discomfort a bit..
what MAOI are you pretending to take, and how much?