I've recently bought a mouthguard for my Karate sparring, but I've come across a problem: How the hell do I correctly mold it to fit my mouth without it making me look like a neanderthal? Is it supposed to make me look like one? Is it supposed to come loose every time I open my mouth?

Instructions on the bag say I should immerse it in boiling water for a set amount of time, then cool water, then fit it in my mouth, but I've done the process about a dozen times and it still comes off every time I open my mouth.

Is there any martial artist/fighter here who could give me their advice on fitting Boil & Bite mouthguards?