If you didn't learn it in school (see my previous comments for rants on modern "designers" who lack design backgrounds), you probably don't know how to do one, or how to take feedback.
Feedback is tough. In the workshops I run, getting good criticism about something you thought up and sketched only five minutes ago is nearly impossible.
What I've found success in doing is three things:
First, make feedback mandatory. Tell people there are "five things wrong with this idea," and get everyone to write down five things in a few minutes. This is pretty easy for people to do: you've said there are N problems, so they already "know" there are things wrong, so they don't feel so bad about it. Come up with five things and say one thing. Have the next person say something different, all around the room until you've covered a lot more than five things.
Second, pit people against each other. Given that list of arguments, have someone (who isn't the original designer) or a group defend the design. If you're a designer, you can figure out why they did things that certain way. Argue against that list of reasons. You'll either strengthen the reasons or disprove them. I've also done variants where this list started another round of designs.
Third, discuss metrics. Have the two groups jointly come up with metrics -- actual testable questions, not just "we'll A/B test," get them to figure out WHAT they are SPECIFICALLY testing -- that will let them "prove" who is wrong and who is right.
I've started doing these things in every workshop, now, including my most recent one on tag clouds. You can see the exercise breakdown here: