I've build a few small lights for vegetable gardens. Three, 40w lights for my winter edible garden/nursery and I just finished a 9w light for a friend to experiment with micro greens. I'm working on a 200watt panel with integrates automation controls.You'll have to do some reading, because there have been a few very detailed thread on this forum and in the DIY section of the forum. The stickies are a great place to start, supraSPL's 'DIY led lights-and how to power them' as well as his 'DIY LED CXA 3070' thread, and his 'heatsinks for DIY lamps' thread are great. Anythreads started by stardust sailor will probably overwhelm you with information about all aspects of light building and how to build luxury quality lights. There are also a host of less detailed, but no less important build threads by various users.
There isn't a step:1 step2: thread.... yet, but the info is easily accessed now. COBS like the bridgelux vero series and cree CXA series have greatly simplified DIY lighting.
To give a few shortcuts:
Digikey is a good place to buy bridgelux and cree chips.
Jameco is a good place to buy meanwell drivers.
Fasttech or ebay is a good place for cheap Chinese drivers and lenses
Newegg/amazon/ebay for arctic alpine 11 GT plus CPU coolers,fan driver, thermal paste, kapton tape, drills, taps, and small parts.
heatsink usa for proper heatsinks.