The following guide is intended for a new multiboot installation using separate disks – with Windows on one disk, and Manjaro on the other. However, you will likely still glean something useful from it:
This may also be helpful:
Cheers.
Edit:- Realizing that you’re currently using only one disk, it might be beneficial to clone it – treating it as your windows disk – which I presume it was originally, and perform a fresh install on a new disk for Manjaro.
From that point, it may be easier just to copy your existing /home content to the new /home after install. I’d suggest creating a new /home partition for the purpose (choose the Manual Partitioning method during install).