if you have the old curtains, use them as a pattern to cut the new ones and remember to allow 2.5 cm seam allowance. Measure twice, cut once (as my grandma used to say...!)
If you want them to be gathered rather than 'stretched' across the windows, then allow about 1.5 x width. Measure each window separately and then speak to someone in a material shop. They will be able to tell you how much material to buy. I'm sure you could also get that info off the internet, too. Curtains are basically just a rectangle with curtain tape at the top edge and about the easiest things to start with. Once you do them you'll be doing all sorts for your van - cushions / bedding / bunting..... I did a campervan shirt for my husband. Great fun!
Don't do as I did first time around. I decided on a simple white stretchy wire system, like you use for net curtains at home. I measured up the material, sewed it all up with neat folded over seams. Looked great until I tried to "draw" them (as in, open them). Of course the material, whilst pretty thin, was too thick to "bunch up" enough. The result was they could only be half opened before the material jammed up at the top (if you follow me).
I made workshop rags out of them and did it properly next time with hooks etc.
Go to a fabric shop that sell the bedding sets.. Duvet cover, 2 pillow cases, and one set of curtains to match.[usually in one packet and cheap] buy 2 sets and you can self line the curtains and make loads of scatter cushions covers.
Use one of the duvet covers and 2 or 4 pillows covers as designed. The rest just cut up and use for the curtains. you should be able to cut the curtains double length, hem all around and then fold in half [making sure you buy a fabric that looks the same upside down] you will have thick curtains [no need for thermal lining] then just sew the tape along the fold [now the top] and hook onto track.. you should have enough to put quite a bit of fullness to make the heading look a bit descent instead of a flat look when closed [at night]
Not only will they look better you will have a complete matching look to the inside of your van.