My process:
Added Rigify metarig.
Deleted left side of the rig.
Positioned right bones.
X mirror bones.
Flip Names of mirrored bones.
Turn on X Mirror and ctrl+R to align both sides.
Tab out of Edit Mode and select Rigify Generate Rig.
Also, you don’t have to mirror bones and flip names. Select the bones you want and do Armature->Symmetrize. If they are named .L or .R, it will mirror the bones, fix the roll, and flip the names in one operation.