Quite simple. To create it, you can either use any PCLinuxOS live CD (any version) or, as an alternative, you can back up the contents of your PCLinuxOS installation into an ISO file. Should you choose to do the latter, I'll explain how it works in the next paragraph. If, OTOH, you'd rather just use one of PCLOS's stock ISO files that you can download from the PCLOS website, then skip to the paragraph after that.
Start MyLiveCD by going to a terminal and typing in "mylivecd" (without the quotes, of course). You'll be prompted for the root password, and once you enter that, it'll take a snapshot of your system and condense the whole thing into an ISO file. Be forewarned however, that it takes a LONG time for MyLiveCD to condense the contents of your system into an ISO file (mine took about 2 hours -- your mileage may vary).
Once that's done, look for a program in one of the menus (just exactly where it is, I don't recall -- I'm at work right now on a VERY slow day, and I'm on Windows and not by choice) called Live USB Creator. Start that, and again, you'll be asked for your root password. After you've entered that, follow all the on-screen instructions to the letter, including when to insert and remove your flash drive. Overall, this phase of the operation takes about 5 minutes.
At this point, you're ready to use your Live USB drive. All you do here is just plug it in to any computer that's capable of booting from a flash drive and turn on or reboot the computer. When you boot up, you'll be given the choice between the ISO you copied to the USB drive as-is or a persistent version of the above, which will keep track of all the files you've placed on the USB drive and other changes you've made since installing PCLOS onto it.
For the record, I keep a 4GB live USB in my pocket all the time that consists of PCLOS Phoenix (Xfce) plus Enlightenment if I should need an even lighter desktop than Xfce. Another advantage to this is that the live USB will be a bit faster than a live CD or DVD -- almost as fast as a hard-drive installation.
I should warn you, though -- there is a bug in PCLinuxOS where, on live USB installations, pressing the number 2 -- namely, the one that, when shifted, generates the "@" symbol -- causes you to go back to the login screen. Someone please correct me if I'm wrong here, but as far as I know, this bug still has yet to be fixed. However, you can simply log in again and, interestingly enough, pressing the same button will not present the problem again. Please note that this problem is unique to live USB installations and does not affect live CDs or hard-drive installations.