NOTE! If you have a BitLockered Windows partition and TPM this may fail in ways I don't want to comprehend. Un-bitlock first if I were you should that be the case.
NOTE! I wrote most of this from memory, one I regularly volunteer is shocking ;)
- Boot into Windows and backup your current UEFI partition using BCDEDIT /export
- Backup Windows if you really give a shit
Before you do anything else
- Turn off SECURE BOOT in the BIOS and make sure it still boots up into Windows