The button opens NeonPlan 3D straight in the HACS of your installation. By hand:
Open HACS in Home Assistant.
Choose ⋮ → Custom repositories at the top right.
Enter https://github.com/Mastershort/neonplan3d, type Integration, and add it.
Search for NeonPlan 3D, install it and restart Home Assistant.
Settings → Devices & services → Add integration → NeonPlan 3D, or straight with this button:
NeonPlan 3D now appears in the sidebar. The dashboard card is available right away, no resource needed.
Copy the folder custom_components/neonplan3d from the repository into config/custom_components/, restart Home Assistant and add the integration as above.
HACS reports new versions by itself. Restart Home Assistant after every update. Until then NeonPlan 3D shows a note at the top that a restart is pending. Your plan always stays, even if you remove the integration and add it again.