That looks like a mortise and tenon joint and I have to admit that I'd got the (wrong) impression that you needed to cut a long groove
Heh - don't think my explanations have been terribly helpful
Now I know what it us you are trying to do I reckon I know how your man made it - he like as not used a square chisel mortiser (or even chopped it out by hand). Frankly, the best way to get something like that cut is to find a local joinery shop - the sort of place that makes replacement wooden windows or repairs sash windows - and ask them to chop out the mortises for you (you'd need to mark out for them, first, though).
That's a deeply appealing option... but I'm determined to have a go. Probably expensive lunacy, but... I just bought a cheap 2nd hand router (½") off eBay for next to nothing, so I'll play around with it on some scrap wood when it arrives. Maybe all I'll discover is that it can't be done this way, but it won't be a terribly expensive lesson.
you are better off using bolts and cross dowels, maybe use a glued tenon joint for the head and foot board, then use bed bolts and cross dowels for the the side rails
Yes, somebody's already suggested that above... it would certainly be a lot easier! But I'm determined to try the hard way