Gesso is a paintable compound used to prime surfaces, expecially canvasses, prior to painting. It gives tooth and flexibility to the surface, allowing the paint to adhere to smooth surface easily. It also fills voids and sands well, so it is great for smoothing uneven surfaces. I especially like to use it to smooth paper mache sculptures.
One of the problems with making your own gesso is lumps. I am going to show you a simple way to make your own gesso with no lumps. Every Time!

Materials

Jar with a lid
Measuring cups and spoons (dedicated to art use, don't use your kitchen cups)
Spoon or something to stir with
Dust mask for pouring the chalk. It does create a dust which can be irritating to some people.
1/4 cup Powdered Chalk (Also called Cacium Carbonate or Whiting)
You can buy it in bulk in bulk at your local ceramic supply store
6 tablespoons White PVA Glue (I prefer Elmers Glue All. School Glue is too thin)
2 tablespoons Linseed Oil (Boiled or raw is fine)
Water

** A note about chalk****
There are several schools of thought as to the best filler for gesso. Some people use talc, marble dust or even drywall compound. Talc and marble dust are often used becuse they are a very fine powder and mix into the liquid portion with minimal lumps. Talc, while readily available in the form of baby powder, has the disadvantage of added fragrances, which could affect the final product. Marble dust is an excellent filler, but it is much more expensive than calcium carbonate. Drywall compound is readily available but often times contain fungicides, which can cause reactions when handled with bare hands or inhaled. My method elimnates the need to start with expensive, super fine fillers and still produces a creamy smooth gesso.

Step 1: Soak the Chalk

This step is really the key to preventing lumps. Take 1/4 cup of calcium carbonate and put it in a jar or container that can be sealed. Add enough water to cover the chalk. Make sure you have at least 1 inch of water covering the chalk, as it will soak in.
Now you wait until the water clears as much as possible. The goal is to have the chalk completey saturated with water, with none suspended. I usually start my mixture in the evening and it has settled by morning. But you can expect to wait at least an hour before the next step,

Step 2:

Carefully pour off as much water as you can without disturbing the chalk sediment. Do not pour the chalk water down the sink as it may cause clogs. It can easily be poured outside as it is a naturally occuring mineral.
You may wish to allow the chalk to rest again and decant once more. You can repeat this step as many times as you like, so that you get fully saturated chalk with no excess water. The more water you remove, the better.

Step 3: Mixing the Gesso

Now add 2 Tablespoons linseed oil and 6 tablespoons glue to the chalk remaining in your jar, and mix well ** Helpful Tip** Add the linseed oil first, it will coat your measuring spoon and make it much easier to measure and dispense the glue
It should be the consistency of a thick cream. If it is too thin, you can carefully sift in some chalk while stirring, but be careful, adding dry chalk may cause lumps. It is best to take your time in the previous step and remove as much water as possible.
The result is nice smooth gesso with no lumps. You can seal the jar and it will keep for several weeks. It may separate over time, but a quick stir will bring it back to the proper consistency.