What method are you planning on using?
You can program them with the plug-in cards, which is incredibly easy, or you have to use external Gateways.
Using on-board hardware for PRI, you just create a Dahdi trunk and designate it g0 (default is group 0), then you just create inbound routes for the DID's coming in on the PRI. Very similar to programming target lines on a BCM. If you were to order the system with a card, it comes already pre-programmed with the trunk. The only thing you have to worry about is whether your provider is running NI-1 or NI-2. The UCx comes defaulted to NI-2, so you might have to change it. You do that using the Configuration File editor under the Tools menu. Click on the arrow in the upper left corner and another menu shows itself. Click on the editor and look for the dhadi-channels.conf file (i think it's about halfway down on page 2) and modify the switch type if necessary (NI-1 is actually dms100). To look at or change the framing and coding parametrs etc, go to System>Hardware Detector. If you have a pri card, you'll see all the channels listed there and there will two links highlighted in blue. Just open them up and you'll see what you can mess with.
If you have a 4 or 8 port analog card, you program them using the PSTN trunk DIDs. It's so easy, all you have to be able to do is be able to read. You assign them a "DID" number and program inbound routes in the normal manner.
If using external gateways, they would be programmed as SIP trunks and the configuration would be dependent on the manufacturer but in most cases, they have pre-set configurations for Asterisk based systems and most work pretty well.
To use the on-board cards, you will need to be using the UCx50e box. The old original appliances can't accommodate them.
