Xeducatex,
Well like I did. Get a cheap digital camera, a large memory card for it and take photos. Lot of them. Get a flickr account and start updating this thread with what you have, questions, plan of attack etc. That will really help you keep track of everything. Use this thread as your build diary. At least that's what I did.
If the bike is in pieces, start organizing parts into the different components. A lot of pieces are going to be foreign to you, so take photos and ask questions.
I would print off all the parts fiche (schematics/blueprints) for the CB400F over at CMSNL. Keep those in plastic protectors in a folder. I printed out all the 'pre tear-down' photos I took for reference, you might not have that luxury. I can provide mine if you're stuck in a certain area (like airbox or cable routing etc).
Buy a manual. Get both the Clymer and Haynes. Get a manufactures shop manual if you can too. You’ll need it.
A space to work in. You’ll be getting dirty and it could be in pieces for a while, so make sure you’ve got decent space. If you have a work bench, all the better. Some people build bikes in their apartment, so that isn’t an excuse.
Tools. These Honda's are all metric, from 8mm to the odd 20mm. I’ve really only needed a socket set, a torque wrench, a set of ring spanners (never use a shifter), a set of good phillips head screwdrivers, a bench grinder with wire cleaning wheel, a dremel tool, an impact driver for stuck bolts, an angle grinder, lots of WD-40, a heavy duty cleaning fluid (failing that, petrol), lots of various grits of sandpaper, a pile of rags and some cold beverages.
Then I would do as I did, strip the frame down, have it and everything else you can think off (indicator stems, handle bar clamps etc) sandblasted and powder coated in a semi-gloss black and start rebuilding.
I gave my engine to a mechanic because I knew I couldn't do it and knew it would take a long time. If you're doing the engine yourself, I'd start with that because you'll invariably have to send off things for engineering and that takes time. Before tearing it down, take tonnes of digital photos of everything. Once you get everything apart it could be months before it goes back together. Some people print out the photos in life size, stick them on cardboard then stick the bolts though the cardboard so they know exactly what goes where. I should have done that.
There are whole threads on rebuild engines (look for Jensen's on the CB400F). Basics like replacing cam chain, valves, cam shaft, valve springs, gaskets, oil seals, bearings, side covers. Get a Dyna S ignition and 3 or 5 ohm coils. Replace points cover if needed.
Suspension/forks. Start with front triple tree (install tapered roller bearings), rebuild front forks (polishing lowers), rear swingarm (install brass bushings and swingarm grease nipple), rebuild rear shocks or replace with replicas.
Then wheels. New spokes, sprocket, rims if needed. New tyres, bearings. Then rolling frame.
Then wiring harness. Get a repo from DSS (davidsilverspares.co.uk) and save yourself a lot of trouble shooting. Then airbox/battery area (use stock setup, forget pods, not worth the hassle). Make sure everything is sealed and not cracked. Mount regulator, rectifier other electronics.
Then lights and switch gear. Rebuild or replace switches, lenes and bulbs. Your headlight bucket will probably be cracked. Fix or replace.
'Tin wear' (tank and side covers). Pull dents out or repair. De rust and 'cream' tank if needed. Radiators places can do this for you cheaply or you can do it yourself. Repaint, apply decals and clear coat.
Seat. Reupholster original or source new pan, foam and cover. Or build your own seat as many have done.
Pipes. Find a good set of originals. They are the unique feature of this bike and 'make' the look. DSS sell repos but they aren't as good in my opinion but will do if you can't find originals in good condition (hard to find cheap these days).
Battery. Get a sealed unit and save your chrome from an overflowing battery (as happened to me).
Petrol. Use only premium and try to find one without ethanol as everyone has a horror story about it. Personally, I haven't risked it.
Start her up and enjoy.
Rick.