actually i am about to make one myself...it's not hard at all...just need metal, a welder, measurements, and a milling machine....
you have to ask yourself a few questions
Do you want to go cheaper on build cost?
Do you want it to "look pretty"?
so performance isn't going to be an issue if you choose the right material from the start.
Carbon steels are an acceptable choice and cheaper to get compared to stainless steels. Stainless Steel usually require an experienced welder and are sometimes much harder to machine than carbon steels. Material grades i suggest for Carbons steels would be 4130, 4340 about 1/4" -3/8" for the flange.
Material grades i suggest for Stainless Steels would be 301, 304, and 409. The stainless that is best for exhaust is probably 409.
However, material thickness for the tubing now is an issue...if you go too thin with carbon steels, it will heat up too fast and fail faster. With stainless steels, if it too thin and heats up too fast, it will contaminate and you run a higher risk of major failure.
So, with carbon steels, i suggest a wall thickness of no less than .1875-.250" of an inch (3/16" to 1/4") and if using carbon steels, it's highly recommended to have a high temp coating also added, ie. Jet-Hot, ceramic coating etc.
With stainless steels, i suggest a wall thickness of about .125-.1875 (1/8-3/16)
now with that....if your turbo can just have a pipe clamped on...best bet is to take it to an exhaust shop that can mandrel bend (not a crush bend) and have them bend it to the right specs for your vehicle.
either way, make sure they are at least 3" inner diameter.
Machining flanges probably can be the most difficult part of the process. Most flanges today are produced via CNC Machining Centers where every flange that is produced is nearly identical. There are places that already have flanges that you can buy which aren't that expensive. But the problem there is what material they are using...if you buy a stainless steel flange with a carbon steel pipe, welding is not going to be an option. Cross welding carbon and stainless steels will crack...they will fail and vice versa.
best thing to do is gather your dimensions straight from the turbo (if making a flange that bolts on to the turbo) and get the dimensions from where the exhaust flange is going to bolt into. Lay that out onto the material you have chosen. Now if using carbon steels and you have a lot of excess metal...best way to extract that template is to plasma cut about 1/8-1/4 away from the said lines that you have put on the metal. (Contour Saws can work but they are time consuming) For Stainless steels this is not an option considering that after you are done, the metal is no longer usable for the simple fact that you have contaminated the entire piece. (Stainless must be melted in an inert atmosphere, aka argon, helium only and since most plasma cutters use compressed air, it will contaminate because of oxygen, carbon dioxide, nitrogen content) So use either the mill or a contour saw to extract it.
Set it up on a milling machine with a good sharp cutter making sure that you know what you are doing and all the safety procedures are followed (no long sleeves, loose hair, NO JEWELRY RINGS WATCHES ETC.) Make sure your setup is strong, and allows for maximum machining per side. Best thing for this and if you don't know exactly what you are doing...take it to a machine shop and have them do it. Machine the piece out using all standard feeds/speeds and de-bur it using a file and sandpaper.
Now test fit it to the turbo/exhaust to make sure side clearances are within what you need them to be. Transfer punch the holes and drill and/or tap the holes...now you have a flange blank.
Before you cut the holes out for the pipe, make sure you have the pipe ready. line it up on the flange blanks and outline them. Cut the holes using one of the ways discussed before (however if you do it with a contour saw, you have to break the blade and weld it before you can use it)...
position everything and get ready to weld.
this is highly important, not following the right welding procedure can cause failure in the part being made.
anything can be TIG (tungsten inert gas) welded, but if using carbon steels you can either use MIG (GMAW, gas metal arc welding), Stick welding)
if using stainless, then go for TIG welding and it will be better to goto a weld shop and have them do it unless you have experience in this kind of welding.
position everything and put a tack weld at four locations....now make sure that this is where you want everything to be before final welding. if you need, break the tack welds and grind them down reposition and when you have got it right...do your final weld.
When you do the final weld, don't do it all at once...instead go in sections to avoid warping. weld one section first, then the opposite, then another section then the opposite. it can be done from 4-8 welds per pipe. after all welding has been completed...wait for it to cool then make sure nothing shifted...
clean the entire thing off with a wire wheel and get ready for final machining of the flanges
set the flange parts up so that the bottom end will be cut (face milling)
take a light cuts until they are glossy and even on both flanges. de-bur all sharp edges...
then send that thing off for coating...
but all in all...unless you have some years of experience, goto a machine/welding shop for this...it isn't something that can be done in your garage at home nor shop beginners try and do...let the experts handle this sort of build.
pictures of the exhaust header i'm currently working on for my turbo setup
i haven't done the final porting on this yet nor the final machining and i used 4130 stock for the flanges with schedule 40 piping for the exhaust routes.