Make double-sided PCBs.

Even with all the tricks to stick with a single-sided PCB, there will come a case when you simply need wires on both sides of the plate.
I have seen different ways to etch homemade double-sided boards, but either failed to align layers well, or the transfer of the toner on one side, smears that on the other side.
So I decided to try the way that they actually make multi-layer PCB in factories - make the different layers separately and then glue them together.
First choose a single-plated PCB as thin as you can find. I use 0,8mm thick.
One advantage is that you can cut it with a scissors.

Cutting thin PCB plate.

Make the two layers as if you are making a single-sided board. Click here to refresh your knowledge of single-sided boards.
Again, remeber that the top layer (the one from the elements's side) should be printed on the paper flipped/mirrored.

Preparation and etching double-sided PCB plate.

Drill the holes and apply glue on the sides that do not have copper wires. The type of the glue is not critical. It is good to withstand high temperature, when you solder the elements.
You can use needles, pins, or anything, with the diameter matching the holes on the board, to align the two layers together.

Glue two layers of PCB.

Align the to layers together and now is a good time to remove the needles, so they do not stick with the glue. Leave the glue to dry.

Align two layers of PCB.

Before you clean the board, drill the holes again, to remove the glue from them.

Double side PCB ready.

You are ready!