Stick the doughnut in the microwave for quick ten second bursts until the jelly runs. Hold the doughnut by the edges and pour out the jelly. Use the baster to get the jelly out that doesn't pour out.
Step 3: Fill
Take the mustard, which just so happens to have a perfect shaped nozzle for this prank, and load the doughnut with mustard. Don't fill it too much though. Then use the baster to put some of the jelly back in to conceal the mustard.
Stick it in the freezer for 5-15 minutes to help the jelly gel again and plug the hole.
Step 4: Place the Bait
Leave the mustard filled doughnut out, or in the bag, with the other doughnuts and wait for your unsuspecting victims to dig in.
Step 5: Laugh and Gloat
Also try other edible substances, hot sauce for example. :)