I've made some cards using the TGS blank cards and so far played one game with them. Obviously I'm never going to be able to test them as well as the official cards have been tested (or even add much as a poorly playtested game). Does anyone have any tips on what to do to make the testing I do as efficient as possible?You could try starting with one of the custom cards in your opening hand, or adding them to your hand mid or late-game in place of a randomly-drawn card from the deck. That way you can specifically test them in best-case, worst-case, and in-between scenarios, and see if they perform at the level you'd expect them to.
I will post the cards in the variants forum for feedback but that's likely to be later in the week.