The most important thing not to forget is to backup (check this out) your Windows 10 BitLocker recovery key if it is turned on.
You would then proceed with installing Manjaro. Manjaro should automatically detect Windows partition and insert appropriate lines into (assuming)
grub2. If you were using UEFI the problem shouldn't exist in the first place.
If anything goes wrong, you can always boot to Windows using installation USB/DVD/CD, choose
Repair your computer ->
Command Prompt and issue the following commands
Then boot to Manjaro, mount your Windows 10 partition and run
check this guide for reference.