It's not actually that critical. If you simply want an in-line joint in the cable, strip each end back about one inch; seperate inner and outer; connect inners in a linear manner (ie keep the cable straight); insulate; wind some thin bare copper wire around cable to connect the braid screens together and maintain the screen. Insulate (self amalgamating tape if outside).
Best thing I found is BNC connectors (similar to those used by old networking cables.)
You can pick them up in Maplins, and just need a middle connecting piece. The advantage of them is that the connections are excellent, and they're fairly resiliant.
Then you can either wrap the whole connection in insultating tape or (if you want to be really posh, heat-shrink tubing.)
Or you could just use chocolate block connectors... ;-)