There have been a few previous tutorials on texture mirroring, but I have found that it is no use unless you know how it happens.

For this tutorial all of the selections will be in the 128x128 format. Refer to the numbers below if you are using a different size.

There are a few key numbers that are involved in making textures. These are the 1/2 mark, the 1/4 way mark and the 3/4 way mark.
If your textures are in:

For the legs,thighs,head
128x128 - 32,64,96,128
256x256 - 64,128,192,256,
512x512 - 128,256,384,512

For everything else we only need the halfway mark:
128x128 - Main part: 0,51,102 Sides:102,115,128
256x256 - Main part: 0,102,204 Sides:204,230,256
512x512 - Main part: 0,204,408 Sides:404,460,512

Here are the different parts:

Legs and thighs:

Making a right leg texture into left leg texture:

Process: Make a halfway selection of 128 width and 64 height from the top line.
Flip vertically

Repeat the process but make the new selection from the bottom.

Before mirroring: After mirroring:

Mapping the legs and thighs:





Chest,pecs,groin,breast,feet,hands:

To mirror the front,back,top,bottom:

Making the right side look like the left side:

Make a selection of 51width by 128 height from the left.
Copy
Paste and move the object so it is touching the edge of what you have just Copied OR make the left edge be touching the 51 pixel mark.
Flip horizontally


To mirror the sides (can be used for hands,feet,triceps and biceps):

Making the left side look like a right side:

Make a selection of 26 width by 26 height from the bottom right corner
Copy
Paste
Move it to just above where you copied
Flip vertically
Flip horizontally

Mapping the sides:

Note: the arrows point up

Mapping the chest,groin,breast:
Remember that you will not see much more than what you see here because the joints will cover up a large part of the texture.

Triceps,biceps:

Making a right tricep/bicep side look like a left tricep/bicep:

Make a selection of 102 width by 128 height from the left
Flip horizontally

Before mirroring: After mirroring:

Also, when using the templates in the torishop, use the chest template for the triceps and biceps.


Head:

Making the right half of the head look like the left half:

Make a selection of 64 width and 128 height
Copy
Paste
Move the object to the right side of the canvas
Flip horizontally

Mapping the head:

As the sphere that makes up a toribash head is made up of polygons, the quality will not be perfect. This is what will happen:
To avoid this, try and keep as little detail as possible in the black area:

Already been made, sorry.