Stir in the contact solution. At this point, you will have very thin slime.
Last, mix in the Model Magic. This can be done much like kneading dough. It will be messy at first, but as you work the model magic into the thin slime, it will come together. Knead until the slime is soft and buttery.
Notes
Whatever color Model Magic you use will be the color of your butter slime.