Recycling tubes as patches works, but its a lot more work and can be error prone.
You cut an oversized piece of tube and dust off all the talc. Then stick a loop of tape on the back side as a handhold for later. Then sand/abrade it really well, in three directions.
Once that's done you spread some proper vulcanising fluid on the sanded area so its got thin but complete coverage in the middle. Then leave it untouched for 5 minutes while you do the punctured tube prep, the same way.
While the tube is drying, use sharp scissors to trim your patch to the ight size. Use the tape handhold and do NOT touch the sticky side. You can try cutting curves but I find a 45 degree corner snip then to much thinner snips off those corners works well enough.
Then if you can, inflate the tube to about where it would be inside the tyre, so abut 50mm for a MTB 2" tyre. Stick the patch on centered over the hole. There should be no part between them that doesn't have dried vulcanising fluid.
Lastly, use a roller, or a dollar coin or something round like the end of a spoon to press the patch firmly into the tube.
Peel off the tape handle, inflate to "firm" which is 5-10 PSI when bare, and leave for a couple hours to test. If it holds pressure that long its fine.
Then roll and stow it, or refit to the wheel.
Upside - Recycling! Save the planet!
No chamfered edge to the patch, so any movement can wear a fresh hole. Hence why it has to be stuck down all over.
Prep time is doubled.
Old donor tubes make for worse patches if they're already a bit ratty.
I once fixed a 20mm gash this way, cos I was broke. That tube punctured again elsewhere but my bodge held for years. I think its still in a spare 26" wheel hanging on the wall...