Asking for set days. How do I go about it?

[mmmmmchocolate]

Hi all I'm hoping someone can point me in the right direction.
I'm due back at work soon after having DC2. I previously worked 3 days, contracted for 20 hours. Mil has DD1 while I'm at work but wouldn't cope with both DC. So DD2 has to go to nursery/childminder. This means I can't work Saturdays as no nursery in my area is open Saturdays. I also want to work set days as I can't chop and change what days DD can attend nursery. As it's costing me a fortune for childcare I also want to drop to 2 days.
So what do I do? Do I just outline in a letter/ email what I want? Tia

Firstly, does your org. have a flexible working policy, and have you checked it?

Next, have you gone on the direct gov uk website and searched, yet?

Finally, I'd suggest trying a search on this site to see what others in similar positions to yours have been advised.

All the best.

Thank you barkis I've just found the gov site and there is an example of how to write a request letter. Many thanks for your suggestion :)