The version for beginners, with apologies if it's a bit noddy:
For the main and dip beams: You will need two automotive relays with 6.3mm Lucas spade connectors, some (10 or more) 6.3mm female crimp connectors, at least two colours of 2.0mm squared cross section area (25A) thinwall wire long enough to reach the relays from a supply of power (see below) and third colour to twice reach to an earth point (I would earth each relay separately, so if one falls off you still have some lights) - probably a meter or so of each power wire and 2 metres of earth wire -, and a crimp tool (see below). Some sticky backed plastic and an empty washing-up bottle are optional.
30Amp relays (for 360W@12V) should do, but a higher rating won't hurt. You can get ones with 4 pins that only close an internal switch connection when powered or ones that have 5 pins and a single pole changeover switch. Either will do. You can get others, with more pins and smaller and bigger connectors, but that complicates the issue. You can also get ones with built in fuse holders, but they'll be blade type fuses and I like to keep them all 6x30mm glass ones so there's only one set of spares. Also, my fuse holders are by the fuse box not hidden away under the dash (there is one under there and I'll find out what it does one of these days).
It may be worth looking to buy wire to carry power and earth that match the standard wiring colours, i.e. blue/red, blue/white, and black.
I would mount the relays close to where the wires run to minimize having to join wires. The main beam is blue with a white stripe and the dip is blue with red. As said, mine are under the dash to the left of the column, and the wires are easily accessed there. I'm nor sure where else is easy.
I assume I don't need to tell you to disconnect the battery earth first.
In each case, cut the wire where the cut ends will both reach the relay when mounted, and terminate both ends with a female 6.3mm Lucar connector. The wire coming from the column switch goes to the top (+ve) of the relay coil. That's frequently labeled pin 85 - there's usually a picture on the relay showing what's what and what the pin numbers are. The wire going to the light goes to one side of the relay switch. If there's a normally open and a normally closed switch terminal, it's the normally open one you want. Frequently that's labeled pin 87.
The other side of the relay coil should be connected to earth. That one's frequently labelled 86.
The other side of the relay switch should be connected to the battery live (+ve). That's frequently labelled 30, both with fused or unfused relays.
If you get an unfused relay I strongly recommend a separate fuse in line with the supply to the relay, one for each. That may mean splicing wires together, as the leads on the fuse holders won't be long enough. I got two fuse holders with crimp terminals to avoid such joins, but it's a personal choice. The supply should be from somewhere that is not switched by the ignition and not fused and fed by a thick enough wire - effectively straight off the battery. In my case that's off the input to the unswitched fuse in the fusebox.
Check it's wired right and then reconnect the battery (+ve first then earth).
With all that done, when you apply power by turning on the column switch or main beam flash, the relay coil closes the relay switch and the lights come on.
As said, I find the big problem with all that is crimping the Lucar connectors on the wires - the insulated connectors, where you crimp through the plastic, are crap. They will work, but reliability ain't their strength. So I'm slowly replacing as many of them on my car as I can with "Uninsulated Female Spade Terminal Connectors".
Also as said a good crimp tool is essential. I have an RS toolkit for the insulated type, and they still fail too often. For the uninsulated ones, I think this is the same as the one I use -
https://www.ebay.co.uk/itm/Non-Insulate ... xyuOtRbOF6. I like it, but it's probably worth getting extra connectors and having a practice to get the hang of it - it's not like they're expensive. There are online tutorials like this one:
https://www.youtube.com/watch?v=_ClY1pssIQs
I know the others will check I got the right values and pin numbers etc.
Graham