In order to start, you need to have Undertale Mod Tool (which you can get here [github.com]) to edit the data.win.You also need to own Antonball Deluxe on Steam.

After you have Undertale Mod Tool installed, press Windows and R, or just type "Run" into the search bar, and paste "steam://open/console" in there. Then, you should be in the Steam console. From there, paste "download_depot 1382320 1382321 918226394225802310" in. You should see something like that. Paste that directory into Run, and then you should have the Day One version of Antonball deluxe. This is the only version where you can play as Snick.

Open up the Data.win in this new depot with UndertaleModTool, and search "Object_obj_player_Other_11". Open that up in the Code section, and scroll down until you start seeing things like "gml_Script_scr_anton(argc=0)" and "gml_Script_scr_danton(argc=0)". They should all be in orange, so they're not hard to miss. Once you're here, replace one of the character's names with "snick". So it should look like "gml_Script_scr_snick(argc=0)". You don't have to replace a character, but it's more complicated to add your own. If you do want to add Snick separately though, there will be a guide at the bottom on how to do that. After you've done this, open up Antonball Deluxe through the .exe in the depot (it doesn't work anywhere else) and choose the character you replaced with Snick (or Snick himself if you added him separately). Once you load into the game, you should be Snick!

Guide on how to add your own custom characters (Snick in this case) [docs.google.com] also where I figured out how to get the Day One version
Where I found out Snick was still in the final game and how to reactivate him [tcrf.net]