The title says it all, I want to make a custom two seat cover. Anyone know of any DIYs or anyone that can write one up?

I have. It's very much like peicing together furniture covers. You markings for you sewing patterns should be exactly the same as sewing a dress together as sewing new auto apholstery or seat covers. There are a few basic steps to follow.

1) remove the seat/s from the car to get a good measurement of the amt of material you will need
2) a cheap way of doing this is use an old sheet to make your marking on...generally pin the sheet together, make your markings
3) use the sheet as a template for your new material
4) Once sown back together, cover your seats.
5) Replace seats back in the vehichle

Note: You'll also never get a good fit unless you remove the seats from the vehichle. If you try to fit the covers on the seat while in the vehichles, they'll fit so poorly that you'll slide in turns and it will have an adverse effect on the driver's abilities and passenger's comfort.

I've got a friend who does it for a living. He's won many competitions and magazine spots from Hot Rod to SuperChevy etc. It's essentially what he's told me to do in the past.