This project was inspired by trikes made by other people. I also purchased plans from atomic zombie, and used them as a rough guide.
Unfortunately, I did not take photos as I went through the build. I will do my best to explain the steps I followed.
I wanted this trike to be made completely out of junk, and I mostly got my wish.

Step 1: Front Chassis

The front chassis, or steering boom, is made out of two identical kids' kick scooters I found in the trash.
The kick scooters had nylon foot plates. These were removed and the frame, which was steel square tubing was cut separating fronts of scooters from rears. The fronts were then placed facing away from each other and welded together.
The front chassis is held to the rest of the frame with bolts so it can be dismantled for easy storage.

Step 2: Rear Frame and Boom

The rear frame & wheel is from an old trashed 26 inch mountain bike.
The complete boom (front & center) is from bed frame angle iron.
Cut 2 pieces of angle iron and weld them in the shape of a C channel leaving a gap as a slot to hold bolts. This allows the seat to be adjustable by sliding it along the center boom. The front boom is another 2 sections of angle iron welded together, again like a C channel but this time with the open side facing down and no gap. The bottom bracket /crank unit adjusts by sliding along this and locks in place using a clamp fashioned in the way shown by atomic zombie's plans.

Step 3: Front Wheels

The front wheels are 20" and taken from a jogging stroller that was also picked up on garbage day. I had used it initially with it's original frame to make a bike trailer. I still have that frame and when I need to use the trailer, I can just transfer the wheels back to it. The axels on these wheels have a groove machined into them. They fit into tubes and are held in place each with a bolt, that screws down to sit in the groove.

Step 4: Seat

The seat & backrest was made from plywood to the dimensions provided by atomic zombie's plan. One of the few things I actually bought was a set of 4 interlocking foam pads from Dollarama for $2.50 total. The pads were then cut to size and glued to the plywood and to each other where I used 4 layers (only seat). Backrest is single layer. The black fabric used to upholster the seat/backrest is from an old umbrella.

Step 5: Handles and Steering

The handles are from the same kick scooters as the front chassis, each handle has one handrest removed and discarded.
Steering geometry was set according to information gleaned on the net and from atomic zombie. Look for info on center point steering and Ackerman steering.
The King pin inclination was left at what the kick scooters provided in fully unfolded position. Caster was set using a protractor and then welding the frame in this position.

Step 6: Drive Train

The crank/bottom bracket was cut from the same trashed bike as the rear frame.
The chain is made from 3 lengths of chains taken from old bikes. The idler pulley was thrown out of a workshop as the bearings were badly worn. I filled it with grease and used as is. It sometimes makes some noise, but for the most part works fine. I've used old garden hose to cover most of the chain that might dirty my pants. Most plans call for routing chain through hose only on the slack side, but I decided to use it on the tensioned side of the chain as well. So far so good.

Step 7: Rear Carrier

The rear carrier is made from the tubing of an old fancy bed headboard. To it is mounted the bottom section of a dog carrier. My dog fits in this nicely when he gets tired of running alongside the trike.

Step 8: Conclusion

Most of you must have noticed 2 flaws: the trike has only a rear brake and no front derailleur. In order to keep it simple and use mostly junk, I didn't bother trying to incorporate front brakes. That would have involved procuring disk brakes or mounting forks over the wheels, making this project unnecessarily complicated. This trike will be used mostly on the side walk and not driven too fast. The rear cantilever brake has proven sufficient for my needs. I am no racer.
That in itself eliminates the need for more gears. However, the actual reason I don't have the front derailleur is because I had already cut off the down tube from this bottom bracket earlier, when I had used it in another project.
I could have done a better job, but was short on time, as the summers here are gone in the blink of an eye, which is also why I didn't have the chance to document this step by step.
Atomic zombie has some very nice, detailed plans for anybody so inclined. Please note, I am in no way affiliated with the guy. Just looking at my work will show you I am not fit to linger in his circle.
However, I am pleased with this trike and especially proud to have used mostly junk, which would otherwise have taken up space in a landfill.
I hope this information will help someone else to achieve their own recycling goal.