The below steps explains how to prepay the vendors.
Step 1 : Create an account of type other current asset which will act as intermediate account to hold the amount paid to the vendor.
Step 2 : Write a check to the vendor by selecting the account created in Step 1. This transaction implies that we are paying amount to the vendor
Step 3 : Enter the vendor credit by selecting the same account created in Step 1 for the same amount created in step 2. This implies that vendor owes us the amount that we paid via check in Step 2