There are several ways of doing this. The simplest and most inexpensive way is to buy some playing cards and use these in equal piles on each side of the lump of clay as you roll it out. For instance, if you want a fairly thin piece of clay for a simple pendant or earrings, you could put a pile of three cards on each side of the clay and use these as a guide for your roller.

When rolling, press down on each side of the roller over the cards to ensure you get an even slab of clay.

If you are using a texture sheet or mat under the clay to texture the back, remember to put your cards on top of the texture sheet to ensure you get the thickness you want. If you don't do this, your finished piece of clay will be thinner than the cards because the texture sheet will have raised it up slightly.

Also if you are texturing the top of the piece of clay, you should roll the clay out thicker as this texture will reduce the thickness of the clay. Deep textures can cause weak spots in your finished piece.