With your DHCP server running, boot the client machine using the PXE ROM. The client machine attempts to obtain an IP address from the DHCP server and load pxegrub. If successful, it should display a menu of available images to load. Select your option for the OS image. If you don't select any available option, the BIOS image is loaded after 3 seconds. You can also use the arrow keys to select the downloaded OS image.
If all goes well, you should now be running your OS image.