I feel your pain too OP.
NEVER attempt to make a Roman blind until you've mastered the art of cutting a straight line, as I know only too well what the outcome looks like! - and, it ain't pretty.
A trick I've recently learned, thanks to the internet, to get a straight edge to start with, is:
pull a single weft thread from the right or left side of the fabric all the way across the width of the fabric.
This will make the fabric pucker up (a bit like pulling the cords on curtain heading tape).
It is important that you ease the pulled thread across the full width before it eventually snaps.
If you look closely, you will see a fine, flaw line across the fabric width and this is perfectly straight.
You can then use a ruler or straight edge to mark the line ,using a disappearing ink fabric marker pen.
Cut along this line, using big, sharp scissors, and this gives you a straight edge from which you can take all other measurements.
If you're using fairly heavy, close woven fabric, it can sometimes be difficult to see the flaw line you've made but if you hold the fabric up to the light you will see it and then you can mark the flaw line exactly with straight pins.
Draw a line along and between the pins and cut - after removing pins, of course.
Don't give up.
The more you do the more you lern about what NOT to do next time and it's a very useful skill to have, plus, if you can sew, you can save a fortune.