please follow this video:

https://www.youtube.com/watch?v=nZep9-vniGQ

 I found these problems with retargeting animation,

• the rig asset in UE is not changeable, you can just use a humanoid structure, to retarget all other kinds of the skeleton structure, 
• there is no ghost pose option, you always have to open two skeleton side by side and guess where the other bones should be
• assigning of the bones in the rig config should be performed on both source and target skeleton
• the retargeting process is too scattered, and confusing (you can find lots of youtube video which are trying to describe the retargeting process)
• modify pose is not intuitive
• there is this Duplicate and retarget animation window which is not good enough and there is no reason to make it separated(I think),
• there might be some others which I can't remember

I tried to solve them one by one

• the rig config asset is editable you can make it from the source skeleton
• no need to set rig config for source skeleton, the tool is creating the rig config from the source skeleton and set it automatically
• with the ghost pose option no need to guess the pose you are seeing it
• modify pose is more clear by the toolbar menus
• all retarget process is in one single window, no need to go here and there
• easy bone mapping by just clicking on bone visuals