You dont say if its a leather one or the black rubber one? If its leather it would need soaking in oil (I tend to use almond oil as its thin). assume its a rubber one so Id try this. On all the webley rifles and pistols I find it easier to remove the barrel or the cocking arm so the barrel can be dropped down to give you room to work. Hook the washer out with a small screwdriver etc, taking care not to damage it. Then have a look in the recess in the cylinder and see if there is any crap in the hole as people use sealant,glue etc to get these to go in. If there is anything in then clean it out. Grease up your rubber seal and (with radiused side to the barrel,flat side to the cylinder) angle the bottom of the seal to the bottom of the hole. With the barrel down as described you shoud be able to hold the seal in with finger of one hand and with the flat of a small screwdriver push the sides of the seal to feed more and more of it in. Its easier if you do both sides and finish at the top. Once you have it in I would use the flat of a larger screwdriver or small piece of wood etc to then tap the face off the washer to push it back into the hole in the cylinder. That should do it. If not then do it again having sanded or trimmed a tiny bit from flat side of washer. Re fit barrel or cocking arm!