After you create a new account and log in, click the "Add a new proof" button to build a proof. Free accounts are limited to one proof.
If you want to upload a diagram you've created elsewhere for students to refer to and make notes on while they solve, simply drag an image file over to the upload box. Optionally, you can click on the upload box to browse your computer for an image.
Using the plus and minus buttons, add as many steps as you need to solve the proof. You can insert common Geometry symbols and pin statements or reasons as hints to students or to force them to solve in one logical direction.
Click the "Generate proof" button and wait for the proof to upload. You've now created a fill-in-the-blank proof! Copy the page address and send the link to your students. Each time they visit the proof, the steps will be shuffled. Clicking the "Grade" button will give them instant feedback on their solution.