Number one thing is to get the tube clean. All you need to do most of the time is to sand it unless it is so filthy that you are pulling more goop into the sanded area. Then most of the time all you need is to rinse it with water or use an alcohol wipe before sanding.

When sanding make sure if there is a rib or other imperfection that it is sanded down smooth.

That's all you need for a self-gluing patch. Just place it down and press for a few seconds. The tire or rim should keep it pressed against the tube until the glue really sets up over time. But it might fail if you ever swap tires. Self-gluing patches are temporary IMHO. I usually will peel them off and clean/sand and put on a real glued patch on when I get home.

With a glue patch you need to prep the tube the same way but the trick is not put too much glue on. Put it on and spread it out with the back of the tube so it is a consist and smooth coat but not too thick. LET IT DRY THOROUGHLY before attempting to lay the patch on it. It can take 4-5 minutes for the glue to dry. If you attempt to put the patch on before that your failure rate will be very high. Make sure not to touch or contaminate the back of the patch. Hold it with your fingernails at the very edge. Make sure you have it over the hole(s) and centered.

The area you have sanded/clean should be well over 2-times the size of the patch. The area you glue should be over 1.5 x the size of the patch. Don't put the glue on an area larger than the area you have cleaned or you will pull contaminants into the patch/glue and that will be were it fails.

After a day, if you have done the patch correctly the rubber will all be one piece. You won't be able to peel it off. It's just as strong (stronger because it is thicker) than the original tube.