The most common coagulant is rennet, the name for an enzyme which causes the proteins in milk to link together. However, the word 'rennet' is a bit vague. Rennet can mean a 'traditional rennet' which comes from an animal stomach. It can mean a 'bacterial' rennet, sometimes also euphemistically called 'vegetable rennet' which comes from recombinant bacteria (using DNA from veal calf stomach cells). Or rennet can come from a fungus ('microbial' rennet). Using the more general and accurate term 'coagulant', we can add in 'plant' coagulants which might be sap from a fig tree or a milk thistle.
Mix the coagulant into the liquid milk and wait until a gel forms.