Now, before embarking on this rather extensive project, you first need to determine if it is indeed for you. It will, at a minimum, take months of your time, the better part of a thousand dollars, and a fair amount of space to store. A good barometer of readyness would be having completed the following: At least a couple regular armies completely finished, at least a few extensive conversions, and some proper forgeworld stuff. If you can complete those without difficulty, then you are in fact ready to take on something larger.
Step One: Deciding what you want. First off, this'll mean which chapter. If you've gotten to this point, you should have a good idea of the theme of each chapter, so I suggest if you're indecisive, selecting based on which paint scheme you prefer(keeping alternative schemes in mind, of course, if you like successor chapters), as you'll be painting a lot of them. At this point, you should probably fiddle around with a tac marine to set up a prototype model to demo your chosen scheme. You'd be amazed at how much difference can creep in from beginning to end if you don't copy off a template. If you enjoy customized shoulderpads as I do, I suggest deciding on using them at this point as well. They can be pried off later, of course, but extra work is best avoided. Once you've settled on what you want, it's time to get the pieces.
Step Two: Shopping. Your first goal will be to pick out a good store with a nice discount. A 20-25% discount'll add up rather impressively over the course of this project, so no point paying retail unless necessary. I intend to post a comprehensive review of discount shops later, but for now, google will help you find a few. For basic materials, you'll need the following: Snips/wire cutters, fingernail clippers(better on round edges), a nice variety of brushes(get decent ones, you'll get longer wear out of them), about a case of primer, and a ridiculous amount of the base color tubs. I haven't kept precise track of the paint in my project, but it's my experience that a company will burn out roughly two cans of primer, and 4-6 GW sized containers of your main color. You'll also want about a container of chaos black, and one of the metallics as well. Of course, using GW colors is definitely not mandatory; You can use some of Army Painter's paints instead. Colors more intense, and saves me a few bucks, but use what you like. You'll also want to order a lot of greenstuff. Don't bother getting this from GW either, as they have a huge markup on it. Search for kneadatite, in volume. You will also need about a bottle of plastic glue per company. For reference, a chapter has ten companies, plus a bit more.
Step Three: Chapter Composition. For a codex chapter, you'll need one terminator company(first company), one scout company (tenth company), four battle companys(2nd through 5th companys), and four reserve companies(the rest). In addition, you'll want a command element. A line company consists of one each of the generic HQs, 6 tac squads, 2 assault squads, 2 devestator squads, and an optional array of other support items. Two reserve companies are all tac squads, one is all assault squads, and one is all devestator squads. Dreadnaughts are relatively rare(figure, no more than two per company), and bikes/landspeeders take the place of assault marines. Armor varies immensely. Landraiders are almost always terminator exclusive. Rhinos or drop pods are typically available for any infantry unit, with razorbacks for command squads. Before I continue, I should mention that all non command squads are typically 10 strong. The amount of vindicators, whirlwinds, and predators is very poorly defined, so feel free to be flexible on this point. Of course, if you decide to make one company have all drop pods, go all the way, and assign it no armor other than dreads, and give EVERYONE a drop pod. Themed companies look great.
Step Four: The Models. Now that you bought enough glue to have your local shop suspicious of you sniffing, buy a battle companys worth of models. The battle company box set is an excellent way to get this, and if you know a local ork fanatic, suggest to him that if you both buy Black Reach sets, and swap models, you can both get a great deal. Now, black reach is elite heavy, so you won't be able to rely on it for everything, but you should probably keep buying these and swapping the orks out until you stop needing more of something(most likely dreadnaughts), or run out of ork players to trade or buy the extra bits.