Step 1: Click on Item you want to purchase
Step 2: Select a plan then click on Sign Up Now button. After clicking on it, you will see View Cart button at the bottom-right corner -> Click on it.
Step 3: On Pre-Checkout step, You can enter a Coupon there (If you have it) -> Click Process To Check out
Step 4: Finally, complete all required information on the Checkout step and purchase the Item.
Step 5: After purchasing the item, please read How can I Install my theme to setup your site.