Lots of ways to tackle that.  If your going to be doing a lot of saddles on smaller diameter pipe you can get a saddle jig just about anywhere (they come in steel, plastic or paper) you can get one for as little as $5.  Lots of reference material out their like (the pipe fitters blue book). You can get a 2 1/2" hole saw and use a drill press.  You could google curve o mark or lees lot's of stuff out their for this it's pretty basic.  If your new to welding and have a real interest in it my advice would be to go hang with some welders or get a job as a helper, welders are a different breed might take more than one try to get in it's a pretty tight club right now with the sortage of oilfield work.  Might hit a few welding shops

Not trying to sound like a jerk but if your not familiar with saddling pipe you might want to get someone with a bit more exsperience to look your welds over.  People put a lot of weight on a-frames, easy thing to get in a lot of trouble with if it were to bust a weld and come down on some one.  Sometimes it's just as important to know what not to weld! With out good liabilityor production insurance things can come back around to bite you in the arse!

welded an axle back togather on a combine one summer,  it busted right in the middle of the field.  I didn't really have time to mess with it I was plenty busy welding dot lines but the guy was in a bind couldn't get anybody else.  So we go out on a sunday to work on it and my helper lit the stubble on fire under my truck, in a panic he jumps in it and stomps on the gas the exhaust blew the embers farther out in the field.  long story short just about burnt my truck down and a three hundred thousand dollar machine to the ground over an $800 repair.  Sometimes it's just better to say no,  same goes for ability when your starting out.  You can do a thousand good welds and no one will know your name but you have one catostrophic failure and you'll be famous!!

Me, I'm not a pro but if I was in your position. I made a simple jig for my drill press to notch pipe to do exactly what your wanting to do. Here is a link to the post. The picture of the jig and hole saw is down the page, you'll see it.

http://www.americanweldingsociety.org/cgi-bin/mwf/topic_show.pl?tid=22425

If you don't have the drill press here is what I would do. You know that the center of your arch needs to be the same at the highest point right? You could wrap a tap measure around the pipe and make sure it is flat and square with itself all the way around, kinda like a poor mans pipe wrap. Make a line around the pipe. This will give you a guage for the highest point of your arch. You could even measure it out, your pipe will have a 1 1/4" radius, so you could measure up 1 1/4", use your tape measure to make a line around the outside of the pipe at that point. Then take your grinder and slowly start working away at it. Your center arc will have to be pretty even, but your not building the space shuttle either. You want it close enough so it fit's well but not like an engineer or cwi is gonna come by and kick you off your own project.

Me, I'd dig in real deep at the center on both sides then slowly taper the arch in with the grinder and check it frequently with a piece of the 2 1/2" til it fit well. A little gap between the two pieces will be fine, then you can get full penetration you just don't want a gap you could park a dump truck in.

I think Tractor Supply has a template made from steel, seen it there before, not sure what size though, 2" or 2 1/2". It's not as hard as it sounds, just nibble a little at a time and work it out. It's good for the brain to figure this stuff out.