Undertale Connect

Method 1 (Recommended)
UTDR Mod Manager https://gamejolt.com/games/utdrmodmanager/896341

1.) Download UTDR Mod Manager, open UTDR Mod Manager.exe and point it to your Vanilla directories
2.) Select "Import" at the bottom of the window, and select the Undertale Connect .zip file (DO NOT unzip!)
3.) Select the mod and hit "Play"

Method 2
DeltaPatcher https://github.com/marco-calautti/DeltaPatcher

1.) Download DeltaPatcher and open DeltaPatcher.exe
2.) Click on the folder icon next to "Original file:" and select Undertale's data․win
3.) Click on the folder icon next to "XDelta patch:" and select the .xdelta file included in the Undertale Connect .zip
4.) Click "Apply patch" and wait for the confirmation window
5.) Move every file (minus the .xdelta and .mm files) into your Undertale directory and replace files as needed
6.) Launch UNDERTALE.exe

Port 6666 will need to be forwarded to host a server.