Now slip your knife into the sheath, making sure to seat it all the way in. Then begin squeezing down the leather around the knife, forming the sheath right around it. You can try pulling the knife in and out of the sheath a couple of times to get a feel for how it is fitting.
I normally only leave the knife in the sheath long enough to conform the sheath to the shape of the knife, and then remove the knife and hang up the sheath to dry well. It is particularly important to not leave the knife in the damp sheath if your knife is not made from a good stainless steel.