Hi, I've recently made a fairly big roman blind (110 wide by 240 long), and used a great kit from Terry's fabrics which worked beautifully and looks great https://www.terrysfabrics.co.uk/prod/blinds/roman-blinds/deluxe-pvc-cassette-roman-blind-kit/

They are quite easy but a bit time consuming. I would get a book out of the library that shows the process in step by step stages. Another tip would be to use a hemming stitch to do the edges, you will get a much better finish. I think I have made about 17 of these over the years and saved a fortune.

Spend time working out the spacing between the battens, if you have patterned fabric ensure its matched! I had nightmare fabric see the photo.
the long blinds have dropped, I think due to extra weight, so the pattern doesn't quite match!
I used rufflette Roman blind tape.

For my first roman blind I used a roman blind kit from Dunelm, worked out great. I then used those instructions to go on to make a further 12 (never buy a house with 5 sided bay windows!) and purchased the bits needed from terrysfabrics as mentioned by Catherine Byrne. They are really straightforward. The key is to take your time with your measurements/calculations and with ensuring your edges and rod pockets are straight.

Hi Lynn, you might find some companies - ourselves being one ;-) - will make roman blinds out of customer's own fabric. It might be worth it if you have limited fabric and you don't have room for error - as Room by Room said measuring is probably the key part of the process here.