You're not stupid for asking. How else can you learn?
The mortise is the hole the old lock fit into. Inletting is making that hole the correct size.
You will have to remove a little wood so that it fits without binding. The trick is where. Are you equipped to take the lock apart? If not, you'll have to do it as one unit..
Find something you can coat the backside of the lock with (grease, Vaseline, or similar substances) that will indicate where it is hitting as you begin setting the lock into the mortise. Then slowly, methodically remove that wood..just enough so that the lock will eventually drop further into the mortise. An exacto knife works well...the type you can find in many craft isles...narrow round handle..thin triangular shaped blade. It's not as bad as it seems once you start. Just go slow and watch where it is hitting, then remove that wood slightly. The new lock will fit eventually.
Sorry for not being more detailed in my response. Curly explained it well. I would avoid anything greasy as your indicator. Not that it wont work but i hate the mess that i always make with it. I have found candle soot to work very well and not half the mess. Just take any cheep candle and let it get burning good. then take the part you are working with and hold it in the flame till it is blackened (dont hold it there till it gets hot) the soot will transfer to the inletting surface and indicate where wood needs to be removed. After complete clean the parts up by spraying with WD-40.
It has been a while but if i remember correctly the main area of wood removal was the main spring and a very small ammount around the lock plate, 1/6th or less.
Good luck and there is no such thing as a stupid question
If you find yourself thinking that a Dremel Tool would remove the wood more efficiently, sit down quickly until this dangerous thought goes away! As suggested, go slowly, removing just the blackened spots only. Finally, it will fit perfectly into place. Good luck!
Thanks guys the info was just what I was looking for. Too bad I couldn't find a you tube video to help. Thanks a lot guys.
No problem! As Berks said, if you feel frustrated, walk away from the project for a while.
The very last thing you will cut out is the border of the new lock...the lock plate. Once the internals of the lock are in place in the mortise, the plate will show you where it hits when it makes contact with the stock. Hold the plate steady and with the knife, cut along the border where wood is required to be removed. Be sure to angle the point of the blade inwards and tight to the metal plate. The knife handle will be slightly angled away from the lock plate when you do this. This will give you a nice tight wood to metal fit. It can be helpful at this point to file a slight
bevel on the plate inwards towards the mortise. Not so much that the visual side of the plate is altered. This bevel can aid the fitting of the lock plate to the wood.