Step 1. Select a 1st parent

Click on the "None" button to switch to Parent 1 selection. Click on any of the blocks you see to pick the first parent.

Step 2. Select a 2nd parent

Click on the mode button again, while it's in Parent 1 selection to switch to Parent 2 selection. Click on any of the blocks you see to pick the second parent.

Step 3. Build

Press the Build Block button to build a combination of the two selected blocks. Click and drag it to the desired area.

Step 4. (Optional) Sell

Click on the mode button while in Parent 2 selection to switch to sell, which will allow you to sell any of the blocks you've made. If you don't want to sell anything, click it twice instead of once to return to the None button