1. Open the Creation Kit application, then go to File-> Data. 2. Check the box to load Skyrim.esm and set your .esp file to active plugin (if you don't have any .esp created yet, just load Skyrim.esm and then save to create active plugin).
3. Navigate to the Actors list in the Object Window
4. Right-click within and select "New" from the context menu, which launches the Actor menu.
5. Choose an ID, for example, "NewFollower," as well as a Name, Short Name (used for dynamic quests), Race, and check "Female" if you want to make the NPC female (the default is male).
6. Check the "Unique" box and "Essential" if you want to make the follower Essential (cannot be killed), or "Protected if you want to make them Protected (drops to one knee when health is low and can only be killed by the player).
7. Navigate to the Factions tab within the Actor menu.
8. Right-click within the box labeled "Factions and Ranks" and select "New," launching a new box with a list of factions.
9. Filter the list for "follower," select "CurrentFollowerFaction" and press "OK"
10. Repeat steps 7 and 8, this time selecting "PotentialFollowerFaction" 11. Click on the "Rank" entry for Current Follower, press F2 to edit and change the rank from "0" to "-1" 12. Navigate to the "AI Data" tab on the Actor Menu and change "Assistance" to "Helps Friends and Allies" in the dropdown box.
13. Navigate to the "Inventory" tab on the Actor Menu and select a Default Outfit. You may need to click on the Preview check box below in order to make the Preview window update with the selected outfit.
14. Right-click within the "Inventory" list box in the center of the Inventory tab and select"New" in order to add additional inventory items and weapons.
15. To modify the character's face, body and hair you may click the relevant options on the tabs "Character Gen Parts" and "Character Gen Morphs"
16. When satisfied with the appearance, click "OK" on the Actor Menu.
17. Within the Object Window, navigate to Character->Relationship, then right-click on the list of relationships and select "New" opening the Relationship menu.
18. Choose a relationship ID (for example "NewFollowerRelationship")
19. Change the Parent NPC from "NONE" to match the ID of the follower NPC you created above (for example "NewFollower")
20. Change the Child NPC from "NONE" to "Player"
21. Change the Relationship Level from "NONE" to "Ally" and click "OK"
22. Within the Cell View window, choose a cell where the NPC will be based (for example, DragonBridgeFourShieldsTavern) and double click to open the cell in the Render Window.
23. From the Object Window, drag your new NPC follower (from the Actors list) over to the Render Window.
24. Save your plugin and test by launching Skyrim and visiting the cell in-game. If you don't currently have another follower, there should be a dialogue option to "Follow me. I need your help."
Here is an excellent video by Best In Slot walking you through the entire process: