I use the “not-so-yummy” recipe from this site, because it’s quick, easy, and I always have these ingredients, but there are a bunch of recipes here, so choose the one that works for you.
Old tshirts, smocks, or aprons are a good idea.
Big paper works well, like the big sheets of newsprint people use for moving.
You can also use cookie sheets or plastic trays, because the fun of fingerpaint is really in the process, not the product.
I’ve done it inside, at the kitchen table, over a tile floor, but I recommend it as an outside activity.
It takes just a few minutes to mix up the paint. Separate it into small bowls and make two or three colors. The fun is in feeling the glop with their fingers and mixing the colors together, so keep it simple.