The Mortise and Tenon is a really easy joint that anyone can do with basic tools. To make it fit perfectly, snugly, tightly takes many years of carpentery experience, but a beginner can make one which fits tight and does the job it needs to do.
So first up you need to mark out your Mortise, thisis the bit that sticks out. If you have a piece of wood without square edges like us, then you should use a straight edge, placed along the length of the wood, and take your lines from that. This way your mortis will be square with the wood as a whole rather than just squared from the piece net to the joint, which may be a bit off. Mark all faces where you want to cut before you get your saw, to make things easier for the cutting to come.
Next take your saw and cut the front face of your wood, to the depth required. If you do not plan on going all the way through with your tenon then your mortis should not be more than half the width of the wood you plan on inserting the mortise into. So as we have here, the windowsill with the tenon is 7cm deep, so our mortise is 3.5cm long or less.
The space you have around your mortise is up to you, but bare in mind that if it is too thin it has a chance of breaking when you are working it into place. If it is too thick then it wont have a lot of support on its edges when standing in place.
Tidy up any edges with a chisel and then you can move to your tenon
Next mark on your tenon. Unless everything you are making is standard sizes then you should map your tenon using measurements from your mortise.
Use a drill all the way down to full depth. It doesnt matter if you go a little bit too far but try to keep it as accurate as possible. Better to go slightly too far than not far enough and end up having to try to pull the wood apart of shave a bit off the bottom of the tenon.
Use a chisel to take away the rest of the wood in the tenon. A chisel should be used for as little wood as possible, which is why we drilled out most of the tenon first.
Lower your mortise in carefully, depending on how hard your wood is, you may have to have a few goes at this, shaving off a bit more of the tenon. Just take it slow as one of the worst things you can do is make the mortise too small and have a loose fitting joint.
And finally push and wiggle your mortise in to place, brace your wood if necessary and continue your build. Good Luck