Casts are one of the main items in Tinker's Construct. Without these you won't be able to make anything very useful.
To make casts you must first make a stone version of whatever item you want to make a cast for. If you want a cast for a pickaxe head, you'll need a stone pickaxe head first. You then place the stone part onto the Casting Table. Next, you pour the Molten Gold/Aluminum Brass over the stone part, creating a cast in the shape of that part. Remove both. You should get the stone part back and a cast for that particular part. It should be noted that you can use blank casts on the stencil table to create ingot casts. You can also use a single seared brick to make the ingot cast in the casting table.