If when you enter a donation you know that it will become a package consisting just of itself, then after you save the donation, click the Package this donation button that you'll find in the green success box at the top of the page. You will be taken straight to the Items > Packages > Create Package page and all the fields that can be filled out based on the donation's info -- title, description, donor, FMV, bidding parameters, category -- will be pre-filled. You can then complete the package info -- add an image, make adjustments as you see fit -- and save. We call this Quick Packaging.
You can also Quick Package a donation that has already been entered. Just click on its name on the Items > Donations > Manage page, scroll to the bottom, and click the Package this donation button.