The coupons you redeem can be used to off set your purchases of mining contracts.
To do so,
Step 1. Login to your Hashnest account at www.hashnest.com.
Step 2. Check your coupons in your account page.
Step 3. Then go to "Mining", choose the mining contract you would like to purchase, press "Shop" then place your order.
Step 4. Choose your preferred payment method and from your coupons.
Step 5. Click “Pay” to confirm your order.
Final step. Wait for your hashrates to be allocated to you and enjoy your mining!