I didn't find it really difficult. I'm not sure how everyone does this, but my steps:
1) Get someone handy with a soldering iron to attach an IDC-10 socket. I can solder big wires but anything finer details like a PCB I got an associate to do this for me. These routers all seem to have the 2x5=10-pin spot for it, it's just a matter of someone fitting a connector to that spot. The connector is 50 cents at Fry's, and if you have a friend handy with a soldering iron this part is something I think it's good to do just in case. If you think you will need it rarely I would probably use a "straight" connector so you open the case when you need it. I chose a right-angle connector and cut a hole in the plastic so it sticks out and I can hook up more easily.
2) Order a MAX233 kit from somewhere, these guys are popular:
I contacted them and asked could I get it already fully assembled and they said extra $6 for all soldered up so I bought that.
3) The only bit of real work I had to do, buy some connectors and some ribbon cable and make one cable.
I used the cheap plastic connectors that just squeeze onto ribbon cable, again to avoid fiddly soldering. So I used a
couple of IDC-10 females to give me my link to the MAX233 kit. I just hooked everything flat at the router end and did all
my wire-flipping at the MAX233 end of the cable to give me the right pin at the right places. I hook the bottom row
of the IDC-10 into the MAX233 if I want ttyS0 and the top row if I want ttyS1. I only needed a single serial port really
I just played around with the other one a few times although they do have a version that will give you both at once.
The other side of the MAX233 kit is a standard DB-9 serial port so cables for that to your computer are easy to find.
I have read you can also use certain Nokia cell-phone adapter cables and just butcher up a new end for it so you end up with a solution
that will go straight into a USB port on your computer, look around for that if you are interested.