You use a translation! Using translations when learning sentences is fine, as long as it's the first time you see them.
The Fluent Forever rule is that no translations are allowed on flashcards, which means the things you review every day can't have English. You need to add enough image-based cues so that you don't need to see that translation ever again.
Don't be afraid to try to figure out the word via context, either. If you know most of the words in a sentence, you can frequently figure out the meaning based on the words around it.