Hi Miguel,
For multiplication you should use between 3mg/L and 5 m/L of BAP depending on what type of banana you are trying to tissue culture. Your other ingredients look fine. Just wondering how much sugar you are using. 30 grams to 40 grams per liter is what you should add.
If you do not have a flow hood or don't trust your clean area, do the cutting under a weak solution of bleach after you disinfect the cutting in a strong solution of bleach (strong solution = 1 part bleach to 4 parts water). For the weak solution you can use 1 part bleach to 20 parts water.
Trim the plant under the solution of bleach and then rinse with autoclaved distilled water. As long as the piece of banana is under the weak solution of bleach, you can trim it anywhere, kitchen table, desk, etc. just don't take too long to do it. The transfer into the autoclaved water should be done in a clean box. Since there will be a slight coating of bleach on the plant during transfer your risk of contamination will be small if you do it quickly.
Here is a video that shows you what to use for your explant.