Every entry on Linguin has an Add to My Dictionary button. It used to keep a list. Now it also builds the deck your Exercises deal from.
Say you run into sobremesa and want it in your Spanish. Save it, and it turns up in your next round. Answer right and it returns days later for another check. Answer wrong and you'll meet it again before the session ends. It keeps coming back until it's truly yours.
Your saved words go first. When they're done for the day, the round tops up with new words worth knowing at your level, so there's always something to practice.
All of this works before you have an account. If you sign up later, your words and your progress come along.
Met a good word today? Look it up and save it.