You got connectors? And a decent crimping tool? (One of the ratcheting ones - the others are pretty useless) Ideally, you'd also have a soldering iron, some heatshrink and a heatgun.
I took the feed for my relays off the blue/white wire coming from the light switch on the stalk. I've got three lighting relays (spots on front and on roof) and they're all mounted behind the fuse holder. Run your switching wire from where it's spliced into the light switch, through your new switch, and down to the relay. The other side of the relay switch should go straight to earth. At this point, you can test your relay is working (listen for the click) with main beam on.
I've got an extra fusebox behind the passenger seat which all my ancilliaries run off. But essentially the wiring is the same. Take a wire from the battery, through the 20A fuse to the relay. Then take a wire from the relay through the bulkhead, along the side of the engine bay to the front of the vehicle where the lights are.
With not much effort you can make a pretty neat job of it. I wanted to be able to disconnect my spotlights, so I bought some Superseal connectors - unfortunately you need a special tool to crimp them onto the wires. Mr Mad Hat Man very kindly offered to put them on for me. An alternative would be those bullet connectors with rubber "boots" which would give the connector a bit of weather protection. If you're not bothered about disconnecting the lights, then you can ignore this paragraph!
One other thing - make sure your lights are well earthed or they won't perform well.
Ian.