Hi

You need to be able to control the loop record, start, stop, overdub etc... from the floor, so you will need a MkII FBV Shortboard ideally.   Play a few bars of your planned loop to get into the rhythm, then at the appropriate point step on the record button, record your four, eight or twelve bars or whatever and at the end point come out of record by stepping on the appropriate button but keep playing so that the loop can roll around unbroken.  It will take a little practice to get the loop points right, but in essence the previous steps are what you need to do.  You cant hope to properly control a loop from the amp front panel using your hands as that way you are almost certain to create unwanted breaks in your loop.

Nick