I am new to duolingo and don't know how to use it very well...
For the first stage of learning a language, simply use Duolingo's lessons, and keep the skills gold by using each skill's strengthen lesson when they stop being gold.
Click on Words. Use the mechanism to run the flash cards. If you click wrong, the word will be shown more often. If you click correct, the word will be shown less and less.
If the the thing that flips the cards is not there, Contact Duolingo to figure out what is wrong.