Paste your deck list into this page below (Spelling must be accurate!). Formatting is
2 Fireball Guy
1 Rude Dude
Place the downloaded json file onto your computer at the approximate location:
C:\Users\%USERNAME%\Documents\My Games\Tabletop Simulator\Saves\Saved Objects
Restart tabletop simulator and it should appear in your saved objects! Load it into the game from there.