Pick a spot on the muffler that is convenient, then drill the hole the proper size for the fitting threads, then tap it, then install it. I usually have the fitting pointing towards the back of the airplane to make routing fuel line easier and shorter. The metal is thicker there also, more meat for the tap to thread.  If you only have the exhaust holes on one side, decide which way you want the holes to point, and then put the fitting where it points back. You could put a hole on each side, and then just plug the hole you are not using with a short screw or even plug with epoxy. it can always be drilled out and retapped. If you purchased a Dubro 6-32 pressure fitting, that is the tap you will need, and there are instructions under the header card.
  Type at you later,
  Dan McEntee

              If it's from a stock OS muffler, it's probably metric. Take the nibble to the hardware store and try a 6-32 nut on it. If that doesn't fit, then try some metric sizes, most likely M3 or so. What ever fits, that is the thread and tap you will need.. If you don't have any taps for threading, you can always just drill a hole for a short length of tubing and JB -Weld a short length of tubing into the hole.
  Type at you later,
   Dan McEntee

One can get technical , or barbaric , on these .

Some early large thread OS muffler ' pressure take off ' nipples had a LARGE bore hole .

Some aerobatic flyers like a SMALL HOLE , like 40 thou. or a RESTRICTOR TUBE of that bore , in the pressure line .
Or a fine bore ( 1/16 max . ) uniflow or vent pipe , in the tank .

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Zee nipples , as they only go through maybe a 1/16th of metal , can be used to thread the hole , if you get it ' just right ' & the metals soft .
A tap vaugely similar helps ! Useually using the TAPERED part , just enough to start the thread right through , and START the nipple firmly .

The Melability will form the thread . If you dont break a plated brass nipple !
BRODAK do Std Nipples . Some are nutted and only require a HOLE .
In fact if theyre shanked youre stuck with ' just a hole ' . They seem to stay put .

If been obliged  to ' fredem ' this way . And only broken one or two nipples . Coarse threads easiest . Usually threadem close to correct & use the nipple to reform the thread. Lubed . Dont Tell Anyone Tho.
Smacks of butchery , like when it tears out the hole .