Splicing into the connector is the normal way to do it, there isn't much other option. I've considered desoldering the socket on the board and soldering the supply in directly, but this wouldn't be advisable for someone fairly new to wiring and soldering.
Ensure you get +/- around the right way in the cable - there's no hard and fast rule / colour code in there! Test with a multimeter to confirm absolutely that you're connecting + to the inside of the connector, and - to the barrel outside. Turn the supply on for the first time disconnected from the printer, and check the voltage is correct and the right polarity at the connector before plugging it into the printer. If you're unsure at any step, post the specifics here and someone will sort you out. Otherwise, CONFIDENCE! 
I run an E3D through the barrel jack and it seems happy (although it also ran off the stock supply fine). I theorize that the connector is already running a fair bit more current than it technically should, but in general it's been OK for most users. If you upgrade the heated bed at all, use a solid-state relay to get the power off the board (look this up on the forum if you get there).
SD3. Mk2b + glass, heated enclosure, GT2 belts, direct drive y shaft, linear bearings, bowden-feed E3D v5 w/ 0.9° stepper
Smoothieboard via Octoprint on RPi
