You are supposed to target the end of the REM phase.

Try different lengths of staying up to see what works best for you. Start with a short WBTB and gradually experiment with longer WBTBs. Analyze the level of awareness of dreams after your WBTB to figure out which time works best. If a short time works well, which is ideal, you could attempt WBTB multiple times for multiple chances at getting lucid. If only staying awake for a longer period works better for you then you might only have time to do one WBTB.

A gentle wake style alarm would be ideal. You still want to be able to fall back asleep.