O o o o h... this I do know! My mum use to cut our fringes throughout our childhoods with the tried and tested SELLOTAPE method. Wet hair, comb fringe section neatly forwards, apply long single strip of Sellotape across it at the level you want t ocut and use lower edge of tape as a guide. It holds the hair in place so that you can see any straggly bits and nip 'em.
DD will enjoy this herself when she has enough hair to warrant it; it's a winner!