generally speaking most lights will have a removeable lens, removeable bulb housing or sometimes both.

if it has a removeable lens they sometimes have an o-ring. if not then they just have a ridge they seal against. you can use a light coat of grease around the ridge to seal it, or a VERY light coat of silicone adhesive. the silicone seals the best but is a PITA when you have to change a bulb.

if it has the removeable bulb they usually have a rubber boot or o-ring - make sure it's in good condition and replace as necessary.

if it has both then use a lot of silicone and seal the lens tight.