Follow the instructions in the freaking book that came with your PB.
1. Put it to sleep by closing the lid.
2. Attach a USB keyboard and mouse.
3. Attach a monitor. If using a PB with a DVI port, make sure the DVI-VGA adapter is on the monitors cable first, that way the PB can auto-detect.
4. Hit a key on the USB keyboard.
Again, in the book.