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as with anything it takes practice to do these rubbed in jewels. The biggest problem you will find is that if someone has already done it before and they have been heavy handed you have got a real job on your hands. I cant remember the technical names for these tools so lets start with the flat bottomed one you use for moving the brass back away from the edge of the jewel so you can get the jewel out. They come in sets of three flat bottomed and three concave bottomed. The flat bottomed is for pushing that fine lip of brass back over the edge of the jewel. Pick one that fits the jewel but has room to be expanded out, keep it very upright and give four or five twists backward and forwards, undo the thread a touch and then another four or five twists, keep going until you can just see the edge of the jewel. People get lazy and just push out the old jewel, this can damage that fine rim of brass and make it a pig of a job to get the new jewel wiped in. Once you have the old jewel out you need to look at the bottom of the hole where the jewel was resting, if it is at all chewed up it will cause you problems. If it is good then get it as clean as you can. The new jewel needs to be the same height as the old jewel, you can get away with a very slight, and I am talking slight, difference but too much and you will again have problems getting it wiped in. The diameter of the new jewel should be a good fit, a bit of pressure and it goes in, too big and it obviously wont fit or you will break it trying to push it in. Too small and if you do manage to get the rim of brass stretched enough to hold it you can guarantee the jewel hole will be off centre. So you have your new jewel sat in a nice clean flat hole. Now you take the wiping tool( the concave ended one) making sure it is not too big or it wont do anything, and do the opposite but this time you are applying pressure as you are in fact gently riveting the brass over the jewel, you start of with the wiping tool so the outside edge is just a fraction larger in diameter than the rim of brass you are smoothing back over the jewel. A few turns each time and then close it up a touch while applying a little pressure, keep going until you have a reasonable lip over the jewel.
It is difficult to explain without pictures and I have not done one for a while but it takes practice, a gentle but firm touch but the main thing is, like I said at the beginning, if someone has been there before you and carved it up a bit you will not be in for an easy ride. If it is very bad, your only option is to mount the plate on a watch mandrel and turn out the hole to get it tidy, you then obviously have to fit a jewel with slightly larger dimensions.
I hope this help a bit.