According to the MAN website:
'If you are arriving at Terminal 1 proceed on foot to the Flight Transfer Centre. ... Once at the Flight Transfer Centre you will be required to go through security and border control checks. Once you have cleared these controls, follow the signs to Terminal 3 and check the flight information screens for the gate of your onward flight.'

That sounds pretty straightforward. I've read somewhere that sometimes you have to wait for someone to turn up to escort you through the Flight Transfer Centre but since T1 and T3 are next to each other it shouldn't take long and should be quicker than going through passport control with the rest of the crowd, walk from T1 to T3 (outside the building) and go through security in T3.