Please have a look at this YouTube tutorial I made.

First of all, for both methods make sure that in the Outliner your model is placed within the root collection of your rig.

• Appending (creating a copy of the rig):
  File > Append > browse to the rig file on your computer and double-click it > Collection > select the root collection of the rig (the one with only the name of the rig, so NOT the collections rigname_cs and rigname_grp_rig) > Append.
  
  
• Linking (creating a reference to the rig source file):
  - File > Link > browse to the rig file on your computer and double-click it > Collection > select the root collection of the rig (the one with only the name of the rig, so NOT the collections rigname_cs and rigname_grp_rig) > Link
  - Object Mode > Object > Relations > Make Library Override
  - Note that you can’t edit a linked rig. You will need to do that in the rig source file.

Please be aware that in Blender a rig can reference only one action. So when you bring in a rig (through appending or linking) that contains multiple animations,  it will only contain the action that is active in the original rig file.

This happens when you change the amount of bones in a limb to a SMALLER amount of bones. The channels of the bones that have been removed will still be present in the Graph Editor and give an error. You can simply select all these channels and delete them.

1. Allosaur - walk
2. Apatosaurus - walk
3. Bear - walk
4. Bird - fly
5. Cat - walk
6. Cow - walk
7. Crab - walk
8. Dog - idle, trot, run, dig
9. Dolphin - swim
10. Dragon - fly
11. Duck - walk
12. Elephant - walk
13. Giraffe - walk
14. Gorilla - walk
15. Horse - gallop
16. Human - walk, run
17. Kangaroo - jump
18. Lizard - walk
19. Mouse - run
20. Ostrich - run
21. Pig - trot
22. Shark - swim
23. Snake - slither
24. Spider - walk
25. Turtle - walk

Because all the rigs have been setup with export to a game engine (Unreal/Unity) in mind. A frame rate of 30fps is the standard for game animation.

Unzip the zipfile that you download from Blender Market and follow the Install Notes.

Stretchy arms/legs are turned off by default in order for the rigs to be game engine compatible. 

But these are the steps to turn on stretchy arms/legs:

• Stretchy FK arms/legs
  Pose Mode > select the hand/foot controller > Tool tab > adjust the Stretch Length (FK) value.
  
  
• Stretchy IK arms/legs
  Pose Mode > select the hand/foot controller > Tool tab > adjust the Stretch Length (IK) value.
  
  Alternatively, you can turn on the Auto Stretch property. When you move the hand/foot IK controller, the arm/leg will automatically stretch.

Make sure to turn on the Fake User shield for all the actions of a rig.
When you don't turn on Fake User for the actions, it can happen that Blender will remove them when re-opening the .blend file.