How do you get straight seams? Sewing Machines Parts and Sales in Los Angeles

I am new to sewing, probably bit off more than I can chew for my first project. A 52.5″x19.5″x2″ bench cushion. The fabric is a thick yarn type of weave that frays at the edges like nobodies business!

So really have two questions.

1) How do you cut large pieces of fabric to the right size? I am glad I left a little extra on the sides because it wasn’t the best of cuts.

2) How do you sew a straight seam? My fabric seems to wobble and I was never sure where to replace my needle.

I finally wound up penning the thread line on the wrong side and using that as my guide. The extra fabric on the edges is both a blessing and a curse. A blessing because I had a few screw ups and a curse because I can’t just match up the seems and see it to a half inch allowance.

Advice on cutting and sewing straight seams?

Thanks!