2 bottles of clear elmers school glue(other brands don't work well). Then refill one of the glue bottles with sta flo liquid starch from wal mart laundry aisle. Mix and mix and knead and knead. Then refrigerate overnight in a plastic ziploc bag. Then knead more when you take it out to play. If its too sticky add a little more sta flo liquid starch. Optionally you can add food coloring or liquid watercolors when you first start making it. If you want to you can also add mint extract or any food extract flavoring of your choice to make it scented. You can find it near spices and food coloring in wal mart. Always refrigerate after use in a closed up ziploc bag. If its ever too stiff add a little liquid starch and knead it for a while. It will be runny at first but firms up with kneading and then refrigeration in a closed ziploc bag. If you want it to glow in the black light, break open a few highlighters of one color and make highlighter infused sta flow liquid starch by squeezing the highlighter liquid into the sta flow liquid starch and then toss the highlighter cartridges into the bottle. I usually keep one bottle of highlighter infused liquid starch around as well as one plain bottle of it.
Here are some awesome examples of my twin boys and a playmate of theirs playing with silly putty that I made!