When selecting a picture to copy, make sure it is large and clear. You might wish to photocopy or do a computer printout rather than drawing directly on a photograph. You need an image with clear lines and edges - a blurry image makes it difficult to find a line to follow.
Decide on your grid size. If the grid is too large, you'll have to do too much drawing in between each square. If the grid is too small, you'll find it difficult to erase, and it can get very confusing. There is no definite rule, as the size of your picture and the subject can be so varied - but something from one inch to half an inch will be about right. You don't have to divide your photo up mathematically - if the last squares are only half filled, that's fine.