Roll your clay to the thickness desired* (see note below) and lay two pieces close together (on waxed paper!) along their straightest edges.
Lay a ruler near the edge and cut one edge with a definite angle under the ruler, then cut the other with an angle going out.
Gently lift the piece you cut "under" and lay the edge just over the other.
Press down very slightly if needed or roll gently with brayer, then use light friction with your finger across (without pushing down!) to pull the clay across and make the line disappear. You want to avoid making depressions in the surface as they are very hard to get rid of. Friction pulling gently across will do it.
Lay another sheet of waxed paper over, flip it over to the other side carefully and remove the top paper.
Repeat the smoothing process on the other side until you can't see the line.
You can go back to the first side if it is the smoothest, or repeat with a third piece of clay for more width.
You can use this method to add pieces trimmed from ends or edges to even out your shape or add length or width wherever necessary, as the clay will always roll out in uneven shapes. If you are really getting big and waxed paper is not wide enough, use freezer paper underneath - but remember, you have to fit this in an oven!
*Deciding on your thickness:
it depends entirely on your design and your plan. If you are just starting out and doing something simple, try keeping it about the thickness of a piece of matboard or a bit less (you can use scraps as a guide). If you are going to be adding more layers for a multi-level mold, you want to stay quite thin. The very thinnest setting will usually not give you a good smooth clay but about the second one will do fine. If you are doing lettering, as a general rule you don't want the clay any thicker than the width of the strokes in your letters. If you are doing a backing layer you want it as thick as you can get it. More details on this when we discuss designs another day.....
Remember, you can go through the entire process step by step with personal help: teaching policies