The first step in making a squish bag is always adding the thing that's going to make it "squish" How much you'll need depends on the size of your Ziploc bag. I used a gallon bag for this one, with almost an entire container of dollar store hair gel squirted into it. The hair gel I purchased was light blue. You can find one like this, or get a clear one and use food coloring to color it whatever color you would like. To do that add the hair gel to the bag, then a few drops of food coloring, zip the bag and mix around with your fingers from the outside until it's all colored.
After adding all the hair gel I shook in some multi-colored glitter, tossed in some beads, and then for the "Spring" theme added some craft foam insects. Butterflies, dragonflies, those sort of things.
My daughter and her little cousin both really enjoyed this squish bag.
Note: It's a good idea to use duct tape to seal the opening of the bag.