Step by Step -
1. Rig expected character using "Auto Rig Pro"
2. Import source character animation, if requires check "Automatic Bone Orientation", "Force
Connect Children", and "Ignore Leaf Bone" when importing the character animation
Note: Using "Force Connect Children" can sometimes produce incorrect results.
3. Open "Auto rig Pro:remap toolbar"
4. "Source Armature" - Imported FBX animation
5. "Target Armature" - The one we rigged with Auto Rig Pro
6. [Optional] - Remove all unwanted bones that do not create any effect on the final animation.
6a. Select the bone in Pose mode
6b. Then switch to Edit mode & Press x and delete unwanted bone
7. [Optional] - Next press Redefine Rest Pose then align the imported character just like the targeted rig & then press Apply & correct the source pose to A pose or T pose if required
8. Now press Build Bones List
9. [Optional] - Make sure the targeted rig is in FK mode if requires.
10. [This is the most crucial step] - Check, verify & fix the bone list properly, otherwise retargeting will give you unexpected results
11. Find out the root bone and check "Set as Root"
12. Now Press the "Re-target" button
13. Congratulations