Pick a level in each light world that is small enough for you to see bandage girl from the beginning of the level. If this isn't possible, then pick a level that is easiest for you to beat.

If you see or hear bandage girl glitching (it makes a distinctive pinging sound) then touch her to unlock the negative level. Otherwise, kill yourself and see if she glitches on your next attempt.

She has a random chance to glitch every time your spawn, so being able to discover if she's glitching quickly is key. Here are good levels to try it on for being short and relatively easy, with bandage girl visible from spawn on most of them:

 - 1-1 (Hello World) using anyone
 - 2-10 (Johnny's Cage) using Jill
 - 3-6 (The Shaft) using anyone (bandage girl is far up but the level takes 6 seconds)
 - 4-6 (Leviathan) using anyone
 - 5-10 (Grinding Mill) using anyone
 - 6-2 (Schism) using anyone