The very best way is to first use a liquid casting silicone and brush a couple layers over your foot to make a “glove” mold. You can find fast curing versions of this type of liquid silicone . Once that is cured, you will need to make a clam shell mother mold out of plaster. This will support the thin glove mold when you pour your first “master” casting. This is probably the more costly way to do a casting but will pick up every pore and hair on your foot.
Another way would be to use a construction grade silicone from hardware store but mind you this is an adhesive and will stick to your foot if you don’t use a good release agent and it is very thick and not well suited for high detail molds. I tell my customers to mix a teaspoon of water and about 3 tablespoons of mineral spirits into a tube of silicone to thin it out and I might suggest Vaseline as a release agent to keep the silicone from sticking. I would also shave off any hairs for obvious reasons.
This company specializes in casting products and has great “how-to’s” on what I described.