Step 1: Get Inspired.
Step 2: Assess Your Needs
Take stock of your physical dimensions in order to build a suitable frame. If you are over 180 lbs, you will likely need to incorporate beefier tubing than the 1" tubing I used, both round and square mild steel. Or you could add gussets, truss pieces, or use other ways to stiffen the longitudinal aspects of this rather long-wheelbase trike. Some flex feels good over bumps, that's a bonus of a long wheelbase. But undue deflection could lead to eventual failure for heavier riders. Think "custom" frame, and adjust to suit your build. As for length of the cockpit, it's good to have some room for adjustment. At a minimum, your feet need to clear the front cargo deck. If you are of moderate leg length leave extra room for that taller friend who will want to try your machine. You can have some flexibility by making the crank mast and the seat points moveable on their mounts in one way or another. Accomodating a rider with longer or shorter legs than yourself will entail changing the chain length with my current design. Incorporating additional chain tensioners can free you from that, but adds to cost of parts and adds a little bit of friction, noise, and weight to the drivetrain.
A basic starting point is to measure your sitting leg length: Sit with back against a wall with legs straight out, and measure the distance from the wall to your heels. This will define the general seat back-to-pedal parameter. Other things like height of canopy, width of steering bar, etc, you can work into the design to suit your build as you go. With this triangular frame design that narrows from front to back, be sure your heels will clear the side tubes as you pedal.
Step 3: Gather Parts Before Making a Hasty Start
I built all three wheels before almost anything. Having them is critical to laying out the frame. Brakes and other small parts were on hand early too, to help with the design process, and to be sure what I wanted was available.
Step 4: Take the Plunge
Ideally, draw with a magic marker on plywood or even the slab of your garage, use a framing square to make a baseline and a centerline for reference. This will keep your frame from being cockeyed. Work over this centerline during the frame build phase until the basic triangle is tacked together. To start, once you have general dimensions, lay out uncut tubing on the floor of your workspace, study the tube intersections and devise the best method for cutting and fitting them together. This could be hacksawing and filing mitered tube ends, or jigging up a properly sized metal holesaw in a drill press for clean "fishmouthed" miters. (I use a "Joint Jigger" fixture made for this purpose.) Make good tight fits before entertaining weld-up. Think about any mods you want to make, any places where room will be tight, and get clear about any concerns with your build up. Are you a size 14 shoe? Adjust frame to suit.
Some things to watch for: Make front cross bar that connects to the steering knuckles wider than the cargo deck, keep in mind that you don't want the turning front wheels to hit any part of the frame in use. However you will be limiting the throw with the steering components, and have some adjustability here. I used 26" wheels. If you use a smaller size wheel, you can widen the frame triangle with more room before hitting the tire. But I prefer the better gear development and the suspension feature of a larger wheel. Too small of a rear wheel can cause problems with finding short enough spokes to suit a large-flanged internal gear hub. I have the tools to thread and cut custom spokes, but its a hassle. I just don't prefer the smaller wheels in this case anyway. I wanted some ground clearance, ease of climbing aboard, and there are a great many tire choices with mountain bike tires as well. You have choices, think about them and adjust accordingly.
Step 5: Fuss with Drivetrain
Although large-loop chain tensioners exist that release and take up a great deal of slack, I'm using a simple short tensioner back near the jackshaft. This takes up slack when I go from large to small front chainring. I welded up a mount point onto the large jackshaft bearing mount. Seems to work well, although it won't take up enough slack to allow me to alter the crank mast position without also changing the physical length of the chain. I don't mind doing that, and I rarely need to alter the cockpit for anyone else. It seems quieter and less draggy than the larger units. and I'm running less chain with saves weight. (But with this bike car, unlike my racing bikes, it has not been an obsessive goal to be nuts about weight.)
I'm using a fixed cog on the 5/8" keyed jackshaft to drive the chain from the rear 8 speed internal hub. Then a freewheel cog from the jackshaft to the chainwheels. I can play with the sizes of these to fine tune the gear output.
Step 6: Some Brake Opinions
Step 7: Steering Issues
Something different that I did: I engineered a system of cables to pull on the steering arms, with a T bar up front as a pulling point. Being a section of headset bearing, it is smooth and has decent leverage. It keeps steering hardware clear of the cargo deck. My main concern was to be able to tidy up the hardware that runs from the front to the underseat steering bar. Many setups use a one-sided bell crank that runs off to one side, this can be a space problem depending on cockpit design. For me it would have crowded the pedaling leg space. My arrangement hides the pulling device (the cables) underneath the center fore and aft tube. Clean, quiet, light, effective. I needed to make custom brass pulley towers to reverse the cable action, as well as give them a narrow line up. Works pretty well.
In much shorter trikes, often the steering is direct, with the rider pulling on handles rising out of the steering knuckles. This wouldn't work on a long-wheelbase trike, too far to reach. Those steering handles can crowd the cockpit sometimes, anyway.
I used established principles for laying out the angle of the steering arms on the knuckles. (See sketch about Center Point steering.) You can research Ackermann steering for more about all this. Basically it's best practice to have the steering arms pulling on the steering knuckles in a controlled way that helps keep the inner wheel in a turn from fighting the outer wheel so much. There still is some disparity owing to the tighter circle the inbound wheel cuts compared to the greater distance the outer wheel has to cover in executing the same turn event. But the way that the steering arm points to the rear of the "car" is germane.
Other parameters, like caster and camber, deserve some study on your part, if only to enrich your knowledge base. For a slow-speed vehicle, there are differing opinions. Some say too much emphasis is given to the need for these angular dimensions to wheel mountings when you're not talking about a motorized vehicle. Camber, especially if set to splay the bottom of the wheels out, can enhance stability in cornering. But it enforces a weird tire wear pattern, not landing on the centermost and thickest part of a cycle tire. It loads the spokes laterally. If a frame design is pretty narrow (and therefore more tippy left to right) a bit of camber can make some sense. Short, narrow sport trikes more often have camber.
Caster is a fore and aft angle, affecting the "trail" of a wheel from its pivot point as it steers. In a two wheel bike, the head tube angle and the bend or rake in a fork combine to make "trail" that produces stable steering results if done right. It's very noticable if done wrong. A bike steers poorly "no hands" if this dimension is off. (note that steering also suffers during "no hands" if steering bearings are too tight.) Some say caster is less important on a 3 wheeler, but sloppy steering connections or poor weight distribution can introduce chatter if trail is non-existent or backwards. (Think shopping cart wheel, rabbiting wildly at high speed, or when you push the cart backwards. On a shopping cart there is a large amount of caster, plus sloppy axles.)
Anyway, after much study, I incorporated very mild caster, and used NO camber. For my wide front end, long wheelbase, and loaded cargo platform, much that I read said that this could be the way to go. Some folks are hung up on this as they see visible camber on most all sport trikes they encounter, and assume it is de rigeur. But I try to keep my designs in context and don't just blindly follow similar bikes if they aren't truly parallel to what I'm building. In practice handling is very good.
Step 8: Get Out and Ride!
Here I'll toss in a bit of advice for would-be welders: If you decide to tackle this kind of project as a lark, in most respects the worst thing that could happen would be something that didn't go down the road well---except please don't take on welding "as a lark." It can be easier than you thought, at the same time more frustrating than you thought. My main point here is to remind you to "be a pro" as far as safety. Put a sign on every door entering your workspace: "WELDING, knock first." Don't weld in the open in your neighborhood with kids or passersby being exposed to the blinding arc. Ventilate; save your lungs and other organs from the fumes. Wear a good helmet--an automatic darkening one is convenient as all get-out and helps technique as well; you can improve initial landing on the work with these helmets. Learn how to run your particular machine. Some are "hot" as soon as you plug them in, or as soon as you turn them on--this contrasts with others that will not let arc current flow until you actually press the trigger on the MIG gun. Every time you pick up the gun have helmet on and a clear path to the work. It is good practice to have a safe place to hang the gun for when you stop-- assume it's "hot" and don't lay it on the floor or just anywhere between beads, that way you'll never generate an accidental arc. Learn to turn off the switch between beads as a matter of course. (you'd be surprised how easy it is to switch the amperage setting, when you meant instead to switch the box off.) This way if you borrow or use someone else's machine you won't get a nasty surprise from an "always hot" version. Get some good books and search the 'net for welding videos. Others are more expert than me. I just want to pass on some good general shop ethics.