You can easily spend $20 to $50 on a watercolor block. Not being a particularly picky soul, I decided to make one out of some plain old bond paper and some padding compound.
Step 1: Supplies
I picked up a quart of this stuff about 20 years ago. It turned out to be pretty much a lifetime supply.
padding compound ($10 or so for a quart) (lifetime supply)
sheets of paper of your choice. I like plain old copier paper, but something heavier, on the order of letterhead quality paper might be better suited to watercolors. I'm not that particular.
ream of bond paper ($4) (makes about 10 watercolor blocks depending on how thick you make them)
wood or cardboard cut to precisely the dimensions of your paper.
Step 2: Backing
I cut the plywood into two sheets 8-1/2 by 11 inches to match the paper size. Try to get the edges as square and straight as you can.
I have a hard time getting things square and precise. If you are part robot, this may be easier for you.
Step 3: Make a Paper Sandwich
Step 4: Clamp the Sandwich
Then clamp them all together. Tighten the clamps pretty tight without deforming the hard backing. You want straight smooth edges all around
Step 5: Apply Padding Compound
After 20 years, it's still in perfect usable condition, however I had to use a strap wrench to get the lid off.
I stirred it up with a scrap of wood, but I'm not sure I needed to. It didn't seem to have separated or settled at all.
Using a cheap brush, apply the padding padding compound all around the 4 edges of the pad.
You don't need to make it thick really, just enough to touch all the edges of the pages.
I like to leave a few open spaces for sliding the edge of a knife in in case the paper won't come free.
Step 6: Let Dry a Few Hours
Step 7: After Its Dry
You may find that one or two of the top sheets get trashed doing this, but that's okay. Just peel off the damaged sheets, the rest of the pad should be fine.
You may find a few places where you didn't cover the edges with the compound well enough. You can reclamp the pad and touch up that area and/or apply a second coat of compound if that happens. Though, the thicker the coat of padding compound, the harder it will be to remove the finished painting when you're done. A thin even coat is best.
Step 8: Ready to Paint
In the picture I've applied a really heavy wet wash. It wrinkles up somewhat but when it dries it will be almost smooth again (see second picture).
Now you can experiment freely with watercolors and take risks and try out many ideas because it is much less expensive this way.
It will probably work even better with heavier stock paper, such as letterhead stock, but of course, that paper costs about twice as much or more. Still, you can probably get about 10 or more watercolor blocks out of a ream of paper, so it is still economical.