This is a simple, cheap homemade chalkboard made from an old furniture crate I found, with a total cost of around $12 for materials. I've found it invaluable for scribbling down calculations, sketching up various project designs, and just generally planning and visualizing my thoughts

Step 1: Materials

You'll need:

A large wooden board of some kind. I used one side of a furniture crate, which had the advantage of providing an integrated border and a thick, solid structure, but any reasonably flat surface will do
Sandpaper or sanding sponges. An electric belt sander will work just as well if you have one
Spackling paste.
Chalkboard paint. You can get this at certain hardware stores, or order it online. It comes in both paint buckets and as spray paint; either works fine but I personally prefer the spray paint due to it's more regular finish.
Masking/Painter's tape.

Step 2: Preparation

The first step is to sand down the major irregularities in the chalkboard. In my case this included removing several stickers on the board and grinding down splinters at several points on the board.

After this, it is important to cover the rim of your board in painter's tape to avoid spackling/painting the edges of the chalkboard. I initially underestimated the amount of tape needed; you should add at least three concentric layers of tape to avoid cleanup afterwards.

Step 3: Spackling

Now is the time to even out the more irremediable problems with the board. Starting with the biggest flaws, such as any major gashes, gaps, or rough patches, spread spackling paste over the board, and distribute it evenly by smoothly but firmly pulling a gift card or some other flexible piece of plastic over it.

Once this is done, let the paste dry before sanding it off and repeating. Depending on how generously and thoroughly this is done, you many need anywhere between 2 and 4 iterations to make the board fully smooth.

Step 4: Painting

Now that you have a solid, smooth surface, it's time to apply the layer of chalkboard paint. Check your specific brand of paint's instructions, typically they will suggest 2-3 thin coats of paint. Go outside to apply the paint, unless you enjoy your house smelling like organic solvents, and then carefully paint the entire board, allow it to dry, and repeat for as many coats as desired. After this, allow it to sit outside until everything has fully dried.

Step 5: Finishing Up

Finally remove the painters tape and clean up the chalkboard. If you accidentally sprayed over edges, sand the imperfections away until you're left with the original frame. Congratulations! You've finished making a beautiful upcycled chalkboard. Go ahead, write your favorite formula up there, jot down # 23 on your math homework, or draw an inspirational picture, and enjoy your useful new addition to your house.