If you want to do multiple-block printing, you have to have some way to line the edge of the paper up every time, so that, say, the red background printed from block 1 lines up with the black foreground printed from block 2. This is called registration.
Step 1: What a Shoddily Made Jig Looks Like
The wood edges here between the area where the printing block goes and the cardboard "guardrail" is 3/4 of an inch. I want to make some bigger prints with a bigger border, so I'm making a 1" jig and a 1.5" jig.
Step 2: Gather Pieces, Mark, and Sand
I measured 1" on the flat part of the 1 x 2 (which is really a .75 x 1.5, remember) and sanded up the shinyish surface of the masonite. The masonite may be a little too slippery once I start printing, making the block a little slidey, but I can scuff it up or add a coat of rubber spray paint or something later.
Importantish: linoleum blocks are mounted on 3/4" deep blocks, so the .75" deep 1 x 2 works well. It doesn't need to be exactly that deep, but if it's much deeper the paper won't reach the edges of the block, and if it's too shallow the paper will arc down on the edges of the block (damp printmaking paper droops), making the registration less accurate.
(See Figure 2, where you can see the height of the block vs. the 1 x 2, and which also gives a sneak peek at the clumsy finished solution.)
Step 3: Glue, Fondle, and Clamp
Get some paper towels and rip off a couple, because your hands are gonna get gluey.
Press the wood into the glue and feel around the edges with your finger to make sure it's flush with the edge of the masonite. This is most important for the first piece, in the upper-left corner.
Clamp 'em down and tidy up the drippy glue. I use dollar-store clamps. I love clamps.
Step 4: Make the Guardrail, Let Dry If Neccessary, and Make Some Prints!
And look! Here's our special guest, a 1.5" jig I (mostly) prepared earlier. This doesn't need measuring or nails (you line up the 1.5" wide 1 x 2 flush with the edge of the masonite), so I just made a guardrail out of a double layer of strips of cardboard stolen from the back of a pad and glued together. Glue is the strongest material known to man.
As with the 1 x 2s, feel the corners to make sure they're flush with the edges of the masonite. You kinda don't have to, really, but you might as well. I also put in some staples with a staplegun for the heck of it.
That's it! Hope this was helpful.
Your pal El Rey