Boil some water and submerge your washed mouthguard for 30-60s (or more if needed)

Carefully remove mouthguard from hot water with a fork or any other utensil.

Wait ~5-10s for the mouthguard to cool to a comfortable temperature.

Bite the mouthguard firmly for 30-60s. Press the edges closely against your teeth, specially your front teeth.

Place it into very cold water again for to cool it.

Wear it again directly to check the fit. Repeat 2-5 times if not firm enough.