Add the white glue to a bowl, then sprinkle the baking soda on top. Mix with a spoon to combine.
Add the water and food colouring (about 10 drops). Mix until it's completely combined and the colour is consistent. Make sure you scrape the edges of the bowl with the spoon as you stir to get all the glue mixed in.
Add 2 Tablespoons of contact lens solution to the glue mixture, and stir until you have a good slime consistency and it stops sticking to the sides of the bowl.
Keep adding small amounts of contact lens solution, kneading it into the slime, until you're happy with the consistency and/or it stops sticking to your hands.
Your slime is complete!
Video
Notes
You can store your slime in an airtight container at room temperature for up to 5 days. If you keep the slime in the refrigerator it will last even longer!