For more information on what Derek mentioned... take a look at page 13 of the Quick Start Guide that came with your Kreg Jig...
I've included the Quick Start Guide here in PDF form. By utilizing your Spacer block and a SPS screw, you can secure the jig to the workpiece w/out a clamp and drill your holes from there! Good luck.
Here's a bit more information on joining mitered corners with the Kreg Jig: