Brush up on your geometry (a little) and working with fractions (a lot.) It's time to lay out the pleats. You'll need to pay close attention to what you're doing here. Crooked pleats, crooked fabric patterns, and an overall amateurish appearance are very possible if you don't. Also be aware that cloth likes to stretch - even where you'd least expect it to. When you're laying out the pattern, you have to make sure the fabric is perfectly flat on the table, and not stretched in any way. If the fabric isn't perfectly flat, you can cut very uneven edges, to include some very interesting curves, requiring you to start all over and wasting material. As with the vinyl, always work with the face of the fabric facing up - not down.
The first thing that you have to do is square up the edges of the fabric. You can generally assume the factory edge (selvage edge) is straight, but the edge that was cut at the fabric shop/material supplier's store almost always isn't. This is relatively simple to do, and only requires some very basic carpentry tools.
Using a carpenter's framing square and a long straight edge (mine's 6' long, and it's just long enough,) square up the material by first finding the factory edges and orienting the fabric with one of those edges laying close to where you're standing. Lay the long edge of the square along the factory edge, leaving the short edge about 2 Â½ to 3 inches away from the edge of the fabric you want to cut. Get the long edge of the square lined up as perfectly as you can. Now lay the straight edge next to the short edge of the square, running parallel with it. You want the straight edge to touch the square along the whole length that they meet. See the pictures below.
With everything lined up as close to perfect as you can get it, you have a couple of choices. You can mark the fabric by holding the straightedge in place, and drawing a line on it with your chalk - having a helper to hold down the straightedge while you mark the fabric really helps here. Once the fabric is marked, you can remove the square and straight edge and cut along the line with scissors. Your other option is to use a rotary cutter like I did and cut along the outside edge of the straight edge.
If you're using a velour fabric, or some other kind of fabric with a heavy nap, be careful not to move the straight edge while you're cutting or marking the fabric. It's very easy to press down on the straight edge and move it back and forth on the surface of the velour up to 1/8th of an inch without the straight edge actually sliding across the surface of the fabric. That may not sound like much, but it can make the difference between a seat cover that fits and one that's too big or too small. Make sure you and your helper know to press straight down on the straight edge, and don't move it.
With that cut made, your fabric now has a square edge. Now we need to figure out how long to cut our fabric.
Picture 1: Lay the long edge of the framing square along the factory edge of your fabric, leaving the shorter edge about 2 - 3 inches from the edge you're going to cut. Lay the straight edge along the outside edge of the framing square as shown. I know the long edge of the square looks bent - that's the camera lens playing tricks. It's nice and straight.
Picture 2: Looking down the length of the straightedge, you can see how far out of square this edge of the fabric really is. This is what we're fixing. The arrows drawn on the fabric tell me which direction the nap of this particular fabric runs. Yes, it runs from side to side. I hadn't counted on that, but it won't matter much.
Picture 3: The cut edge is now nice and square, which will ensure that the pattern in the fabric will be straight and centered.