I know sweet Felicity Arkwright about traditional boat construction but I've cut plenty of scarf joints in my time. Routing using a jig is entirely possible but a bit OTT unless it is cabinet work you are after.
I would cut a simple plywood template to mark an identical Z shaped profile on the 2" side of each plank, then cut the entire joint on a band saw. You will need a roller stand to support the other end of long timbers (fifteen squid from Axminster Power Tools). The plank will be a bit wobbly because it is standing on its narrow edge while you saw - holding it against a square offcut cures this.
Use the widest blade the saw will take, with fewest possible teeth per inch - 4 TPI skip tooth is good. If you don't have a band saw that will take 5" depth, ask around in the pub. Every secondary school has one, as do local sawmills and amateur enthusiasts, and all are open to bribery with beer. Once, in desperation, I even enrolled on a woodwork evening class just to get access to their machinery. If you do borrow a machine, buy a new blade for the job (only £10 to £15); you get the benefit of a fresh, clean cut and you've done the owner a favour.
Failing that, as Carl points out, cutting the joints by hand really isn't a big job and it's good for the soul. Use a decent rip saw for the long, shallow cut (some dodgy general purpouse saw from Homebase really will be hard work) and make sure it is sharp. Scribble candle wax on the blade if it binds in the cut, and use plenty of lubrication for the operator.
The angled undercut mentioned by others is generally a good thing, discouraging edges from popping up as the wood moves over time.
I seem to recall that Alan Herd was shown using scarf joints on Dover - if he can do it, you will find it a doddle.
Ash