In order to find your Groupon voucher, you'll have to go directly to the Groupon website. Sign in and then click the My Stuff button in top right corner of their site. 

Locate the deal under My Groupons and then click View Voucher

You will then see a voucher code beginning with the letters GP, and you can copy that code to use in the voucher/coupon box at checkout at

With your project in the cart, click Edit Code, paste your GP code, and click Apply Code. 

Your adjusted total will then reflect your Groupon price. Click Check Out to process the order. 

If your voucher has been marked redeemed, you will have to reach out to Groupon support and they can provide that information to you.