Step by step
In order to retarget an animation in Blender you will need to do the following:
Open the Retargeting panel
Select an armature with an animation as the source armature, select an armature that should receive the animation as the target armature and then press "Build Bone List"
Check if the bones got filled in correctly and fix any incorrect or missing bones
Select "Auto Scale" if the armatures differ in size or resize them manually
In "Use Pose:" select the pose that should be used for retargeting
Important: Make sure that both armature are in the same pose for correct retargeting
Press "Retarget Animation"