I used a guide from Deck Wizard:
Easily Dual Boot ANY Steam Deck into a machine that will boot SteamOS or Windows 11/10 via an Custom Bootloader, with ONE press of the power button.00:00 - I...
www.youtube.com
The only difference is I didn't re-build my deck form scratch, I used gParted to shrink the SSD partition down to give me space (I have a 512gb Deck so made a 80gb Windows partition).
Works well, and the dual boot menu makes it easy to switch between OS as required.
Windows isn't perfect, and I think I'll use SteamOS 90% of the time, just wanted windows for those few non-compatible games.