From a discussion here and linked again here (italicized bit directly answers your question):

"Let's be precise, since the words can get a bit muddled. There are major and minor tarot cards (seen in the mission interface), and there are also drop items called Uther's Tarot. Each major and minor tarot card has a Level, from 1 to 5, and they all start at 1. Each also has a progress bar, which initially sits at 0/10. Using the drop item Uther's Tarot adds 1 point of progress to a randomly-selected tarot card which does not yet have its progress bar filled. Once a tarot card fills its progress bar, you can elect to pay some credits to level it up. That card then has its bonus rewards increased. In addition, its progress bar is reset to zero and the number of Uther's Tarot drops needed to reach the next level is increased....

If you're attempting to level up a single card to its maximum (5), then the optimal strategy is to NOT click the level-up button on all the other cards when they reach full progress. Just let them sit at 10/10. This way, when you use an Uther's Tarot drop, the game will randomly select a tarot card to progress from one of the ones that doesn't have a full progress bar. If all your tarot cards have full progress bars except the one you're trying to level, all the progress will go to that card. (Helpfully, if all your tarot cards have full progress, the game will not allow you to use an Uther's Tarot item.)"

Here's a couple other links that discuss it.