Ok I assume you will have no access to any heavy duty or special equipment and will be doing this at home??
Get some 6mm or 1/4" plate or similar.
Use the exhaust gasket or make a pattern for the manifold by using a cornflake packet type cardboard.
Using a ball pein hammer and holding the card on the manifold you can tap gently round the edge and it will cut out the pattern.
Then put the pattern on the plate and mark it out. You can either scribe it or put masking tape on the plate and draw round it or just use an old spray paint can and it'll leave the outline.
You'll need a pillar drill and drill lots of holes round the inside edge of the port then chisel it out and file the hole round with a half round file.
Drill out the holes for the 2 studs then either using a hacksaw or angle grinder cut round the outside of the pattern.
DO NOT CUT THE MANIFOLD OUT OF THE PLATE FIRST AS it makes it more difficult to hold!!
Tidy it all up with a file/grinder.
Obtain some old bike downpipes of the correct size(a four cylinder bike will give you plenty of bends etc.)
Instead of taking it round the front of your engine take it round the back and,as I presume you want to keep your belts clear it can either be a high level straight back ,in which case you will only have one bend welded from the manifold or you can use a couple more bends and take it to the r/h side??
Of course if you have access to a plasma cutter or a laser it makes it easy
