Here's what I did...
First weigh your item - you need to do this so that you use enough dye to get the colour you are aiming at. I rolled this curtain up and popped it on my kitchen scale. For the Tintex dyes that I used - you need 10gms of dye powder for every 500gms of material.
Thoroughly wet the fabric - this step tuned out to be more important than I originally thought. On my first dye attempt - I did bath towels. They soaked up the water so well that the issue that will become apparent further along didn't arise. At any rate, I think a lot of curtains and pillows will have been coated at some point in their manufacture with some sort of fabric protector and will be harder than you think to get wet evenly. (see photo below) A small squirt of detergent may help to get the water into the fabric.
Frugal-ness: 4/5 Extending the life of something with minimal cost - Very Frugal!
Time cost: About 45 minutes a curtain, but gave me an opportunity to clean out that corner while the curtains were down that I wouldn't have taken or cared about otherwise!
Skill level: For the more confident - not a good beginners project!
Fun-ness: Am happier now that they are back up and look OK - but the stress levels were quite high as I thought I had ruined the curtains for a while there!!!!