You can apply a single donation towards a pledge by modifying the donation record type on the donation:
  1. When on the donation record, click Change next to Donation Record Type: Single Donation
  2. Under New Record Type, select Pledge Installment
  3. Click Continue
  4. Under Pledge, search and select the correct pledge to apply the gift towards
  5. Click Save
Your donation should now be listed as a pledge installment for the selected pledge.