The basic ingredients of pastels are simple: pigment, a filler, and a binder. You start by dissolving the binder, mix in the pigment and binder, get the consistency right, then roll out your pastels and leave them to dry. It will take a bit of practice and experimenting, so keep records of what you do so you can recreate your successes!
Cheap Pastels Recipe:
Quarter cup of rolled or crushed oats (combined with the water to make a binder)
A quart (just over a liter) of water (preferably distilled so there isn't any chlorine in it)
Two tablespoons of powdered tempera or poster paint (for pigment). Another option is to collect leftover dust when using bought pastels and use them to make new ones (the mixture of colors can give a beautiful grey)
Half a cup of unscented talcum/baby powder (for filler)
Step 1: Put the water in a pot and set it on the stove to boil. Add the oats and leave it to boil for five minutes.
Step 2: Pour the oats mixture through a fine sieve to strain out the oats. You'll be using the water only.
Step 3: Mix the talc with the paint powder, then add a teaspoon of the strained oats water. You're after a consistency like dough or putty consistency, which sticks to itself not your fingers.
Step 4: Roll out into sausages, put on absorbent paper (newspaper is a cheap option), then cut into pieces about two inches (6 cm) long.
Step 5: Leave to dry at room temperature, at least 24 hours.
If your pastels are very crumbly, your binder was too weak; add some more oats next time. If your pastels are very hard, your binder was too strong; break up the pastel and dissolve it into some more binder.
You could use diluted wallpaper paste as a binder.
Create tints of a particular color by adding more filler or white pigment.
True Pastels Recipe:
Gum arabic or gum tragacanth (binder)
Chalk or kaolin/China clay (filler)
Step 1: Dissolve the binder in the water in the ratio 1:20 (one part binder to 20 parts water).
Step 2: Mix filler and pigment in the ratio 2:1 (two parts filler to one part pigment).
Step 3: Add the binder liquid to the filler/pigment slowly, until it has the consistency of dough or putty.
Step 4: Roll out and dry as described above.
Gum arabic makes a harder pastel than gum tragacanth.
Damar resin mixed with plenty of white spirit or turpentine can be used as a binder.