Mix together 8 oz Elmer's Glue with 3/4 cup water + 2 drops food coloring (optional). Dissolve 1 tablespoon "20 Mule Team Borax" (found in laundry aisle) in 1/4 cup warm water.
Take a Popsicle stick and dip it into the glue, then stir it in the borax solution until no more will clump on stick. Take off of stick and repeat. Mix up more Borax solution as needed. Play with GAK to blend. Store in airtight container or ziploc bag.
If you just want to make the GAK and not turn it into a project, the two solutions can be mixed together in larger portions to make the proper consistency.