Maybe it's just luck, but I have put literally hundreds of patches and maybe 3 or 4 failed. I even overlapped patches, used tubes with 20 patches on them, no problems. My method is simple:
Start with clean hands.
Locate the puncture and draw with a pencil a sort of of crosshairs with the hole in the center, to aid centering the patch later.
Sand very thoroughly with 60/100 grit sandpaper an area about 1 1/2 larger than the patch. This is the step that takes more time, I like to achieve a clean, fully matte area and erase any ridges or molding lines. Wipe clean with the back of your finger.
Apply a thin layer of cement (with my clean finger) and let dry 3 to 5 minutes. It should be fully dry before applying the patch. If in doubt about the tube "patchability", I apply a 2nd layer and let dry another 3 - 5 mins.
Peel the patch carefully taking care the edges doesn't curl and apply without touching the contact surface.
Press hard on the patch - preferably with a roller, but not really needed. You can use a screwdriver handle, or any similar tool, even your thumbs.
That's it. You can use the tube immediatly, in fact probably the air pressure helps in pressing the patch.