If you are determined to do it yourself it's a straightforward job, assuming that the installation can handle it. Is your consumer unit (fuse board) the old style with fuse wire or the newer style with flip switches? I assume the wiring is in flat white or grey twin and earth (modern cable). If the installation is very old then don't touch it.
To go further you need to TURN OFF THE ELECTRICITY! Turn it all off should you mess up and accidentally cut into a circuit you haven't isolated. CHECK THE CABLE IS DEAD - use a non-contact voltmeter or even a neon screwdriver, but always check you are not about to cut next doors live cable in your loft! Be sure that you have found the lighting circuit and make sure you have found a part with a permanent live and neutral as if you tap off a switch drop you won't get the desired effect!
Cut the lighting circuit, strip back and install the junction box, make sure that you use some earth sleeving on the bare earth wire. The simplest way if your wire is ok to go from the JB to the switch then on to the lights is to loop at the switch. In the JB you basically connect all three lives (brown/red) in 1 terminal, all the neutrals (blue or black) in another and all the earths (green & yellow) in the third. One terminal remains empty.
Take this wire to the switch and cut it. Join the neutral from the feed in to the lights out together using a push fit or a piece of connector strip. Take 1 live terminal to one terminal in the switch and the second live to the other. Connect the earth in the 'parking space' in the back box. Then run from the first light, to the second etc.
To be sure you have continuity use some spare T&E to ensure continuity of all wires from the JB to the last light - you can use a simple continuity meter or even improvise using a torch and completing the circuit with the wires. A simple multimeter is invaluable.
