Question about playtesting

So I was wondering, for those of you who make custom cards, how do you playtest them? I would really like to make my custom cards as polished and playable as possible, and I’m not really sure how to effectively playtest. Is Tabletop Simulator my best bet, or is there a good way of doing it in real life? I currently have a Chromebook, and I don’t think there’s a way to play Tabletop Simulator on there yet, so I’d prefer in-person, but I tried playtesting by substituting real cards for my own, and it was pretty confusing to say the least.

A follow-up question: if Tabletop Simulator is the way to go, how do you play Dominion on there/put your custom cards on there?

Thanks!