1. Go to B&Q and get the following items: A kneeling matt ( for the foam it is made of ),
some drainpipe that will fit around your arm, some silver spray paint, a paintbrush, some paints: orange and red, a black sharpie pen, a yazoo bottle, a hot glue gun, extra tacky glue and duct tape.
( note: you may not be able to get all from B&Q alone...)
2. Cut the drainpipe so the it goes from your elbow to about 1" past your knuckles.
3. Cut the foam from the kneeling pad so that it covers the whole outer surface of the pipe.
4. Use the extra tacky glue to stick it on.
5. Study an image of the arm cannon and cut out all necessary pieces like the big buttons on each side that samus curles out and presses a button to call her ship or whatever.
6. Glue on each piece with the glue in the correct places.
7. Cover it all with silver spray paint.
8. Use the orange and red paints to paint on them flames that are on the top of the cannon.
9. Then, use the black marker to draw on the lines.
10. At this stage, feel free to draw on any markings or other stuff to customise it a bit. ( I highly recommend Tibetan script as it looks a lot like Chozo script.
11. Cut the yazoo bottle in half and cut off the neck where the top screws on.
12. Cut out four small strips of cardboard and them stick them on the half bottle with the hot glue gun. ( These form the segments of the end nozzle that unfurl in metroid prime when samus fires missiles
)13. Paint the end nozzle with silver spray paint and fix it to the pipe body at the end.
Note: these final steps are optional!
14. Fix a glove to the rim at the back of the cannon so when you put your hand in it slips in the glove
OR I recommend you fix a soundboard in the cannon uploaded with a beam of your choices sound so when you pull a trigger inside, it sounds like the real thing! 
CONGRATULATIONS YOU ARE THE FIRST STEP OF THE WAY TO A COMPLETE METROID VARIA SUIT!
