You need to remove the stock and buffer tube.
Then, remove the pistol grip.
Push out the pin in the middle
Remove Magcatch
Put selector switch on Semi.
Lift up the GB abit and should give you enough room to remove the feeding port.
Remove the GB completely in order to mod the lower receiver though.
Reference :
http://www.mechbox.com/m4m16/m733-disassembly.html