rachciav, make small portions of bondo so you dont put too much on, or waste too much. I always make a team spoon's worth and just make a small bead size dot of hardener, maybe 2 beads, depends on whatever comes out of the tube.
Make sure you fold the bondo, it takes longer to fold, but you'll have less air bubbles. Mixing it in circles traps air and causes holes to appear after you sand.
You can always mix some up and put it on, but you cant ever take off a major problem. I ruined a helmet the first time I used bondo, way back before there were tutorials on how to use it. I followed the instructions on the can, made too much, rushed to use it all, and covered up so much detail, I ruined the helm.
Just use thin layers, you just want to smoothen out those seams.