To import Blender Model .json files from Creator's Kit to Blender:

1. Download the most recent CK_to_Blender_addon.py file
2. Open Blender, go Edit -> Preferences -> Addons tab
3. Hit "Install." Find the CK_to_Blender_addon.py file you downloaded and select it
4. Enable the Runescape Import addon by checking the checkbox

To use the importer, use the hotkey Ctrl + Shift + I. Select the .json file you exported from Creator's Kit. You can choose whether to Scale Down from a size that Runescape understands to one more appropriate to Blender (scales by 1/128).