Pic 1 shows the under side of the board with the new plug installed. You can see the legs of the new power connector sticking through the board.

Solder the two legs to the PCB as shown in Pic 2 to secure the power plug in place.

You can now re-assemble the hotend to the PCB and the PCB to the platform in the reverse order you removed them.

If you need to re-reference how to assemble the hotend to the PCB and the PCB to the platform, you can follow the SE300 build guide located HERE.