my fabric one i wash in the machine at 40 with a mixture or half washing powder and half washing soda- comes outperfect every time even if the bottom has gone a bit mouldy!
I have to agree with Judiewudie but if you have a curtain you want to keep, put it in the bath, windows open then tip on cream cleaner mixed with thick bleach scrub the mould dirt etc with a brush rinse in bath then wash in machine comes up a treat have done this myself. Hope this helps you.