My procedure;
Place gun on a table on a towel or blanket use a table lamp for good lighting so you can see.
Using a Crown saver or cleaning rod push a small patch up to the groove from muzzle end, (to give oring a wall to rest against and not just push up the barrel), coat oring with a little silicone grease and stuff oring in breech using a tooth pick, the thick kind, start any part of oring in the groove and drag it little by little around the groove with the tooth pick, be patient and take your time if you get frustrated walk away for a few minutes, this procedure has worked for me many times hope it helps, good luck!