What files were you taught with in your training? It's always best to use what you were trained with, I start with a 100 grit file then a softer buffing file but if you're new to nails then perhaps a 100 grit would be too coarse for you as you could damage the nail plate. That's why it's best to use what your trainer recommends. Are you practising on a training hand or a real one?
Oh and don't give up you will get there!
Best way to blend is with a low grit nail file BEFORE you place the tip onto the nail, if you do still need to blend then a 240 grit should be ok. You'd still need to be careful of the natural nail though and avoid filing it.
I'd go with what your instructor told you to do. I was trained always to blend after application with a 180 grit file. I now use pro impressions tips when needed and they need little blending at all but it's still a skill you need to know x