It's quite simple actually.
Basically, as the reaction proceeds you take aliquots of the reaction (however often you want) and run a TLC on them.
On that same TLC plate, if it's possible, you have a spot for your starting material and a spot for your product. So you'll have a spot for your starting material, reaction mixture (RM) and the product.
When you run the very first TLC plate, the RM should have mainly a spot for the starting material. As the reaction proceeds, later aliquots will show less and less starting material, and more product. Eventually, if the reaction goes to completion, the starting material spot should be completely gone from the RM fraction, and it should only show your product (and possibly by-products).