Select a trailer for your project. If you are going used, avoid a bent frame or excessive rust. If the frame is straight and solid, see that the trailer has an operable tongue jack. That will be extremely important as the project gains weight. Also, go ahead and upgrade the wheel hubs to new ones. There is professional help available for this step, at a tire shop, frame and axle specialist, or online forums. Hubs are fairly inexpensive and will offer some peace of mind when towing. It's better to have a dependable undercarriage before you invest much material and labor into the project.

Supplies

Trailer frame. Possibly a new axle, wheel hubs, tires, so forth.

Step 1: Clean the Trailer Frame After Component Inspection. Apply New Paint.

Once you or a competent person has declared your trailer roadworthy, go ahead and clean it in preparation for paint. I used an angle grinder for tough corrosion, and a drill with a wire brush for all other cleaning. Then I bought Rust-Oleum primer and equipment paint and sprayed the frame. I also bought brand-new tires.

Step 2: Frame the Floor.

I apologize for the shortage of photos during this process. I built this two years ago, not intending to publish a tutorial.
Attach your wood flooring frame to the steel frame of the trailer. This is the best time to run your trailer light wires, too, so they will be concealed inside the framing and safe from road and rodent damage. Also consider any other wiring you may want at this point, such as 12 volt DC or 110 AC. Don't skimp on insulation, either. You want your feet to stay warm on chilly nights.

Step 3: Frame the Walls

Decide how thin you want your walls. In consideration of saving space, I didn't want 3 1/2" thick studs. So I bought 2"x6" and ripped them all to 2" for the walls, and 2 11/16" for the roof. Then I screwed and glued all of the framing members together. That is much stronger than nails alone. I used 2 1/2" wood screws, and 3" screws where I wanted a little more hold.

Step 4: Insulate.

I used 2" foil-faced rigid foam insulation between studs. I squeezed Great-Stuf into every gap and crack I could. I was able to get an R13 insulation rating in only 2" walls. That's something fiberglass won't offer. Plus the rigid foam adds structural integrity to the walls. Don't forget to run your wiring throughout the framing process. I also installed my windows at this time.

Step 5: Seal the Exterior.

Make the cabin waterproof. I opted for 30# roofing tar/felt paper. I like the thickness and durability compared to Tyvek or comparable housewraps. I laid this over the OSB roof sheathing and the walls, with aplroximately a 2" overlap. Tape all the seams and around the windows.

Step 6: Begin Siding.

I used rough cedar that I salvaged from shipping crates as siding beneath the back porch and as trim around the windows and perimeter of the cabin. I made my painted siding from 1/8" Masonite that I ripped in 5 1/2" widths. I primed and painted the Masonite before installing. I used reclaimed barn tin for the porch ceiling for character.

Step 7: Add Porch Railing, Exterior Lighting, So On.

I made my porch spindles from pieces of electrical conduit that the electricians throw away on construction sites. I painted them black, then had a fun time making a creative back porch. I installed a small air conditioner up in the sleeping loft above the porch.

Step 8: Finish the Interior.

I lined the inside of my cabin with 1/8" Masonite. It was inexpensive, sturdy, and doesn't weigh a lot. I built a tiny bathroom, a folding sofa/bed, food and camping gear storage, and a tiny staircase leading up to the sleeping loft where I have a queen foam mattress. I also built my own doors - the entry door and the bathroom door. I obtained the screen door from an old camper I had around. I installed a 30 amp breaker box beneath the stairs, as well. The toilet does not empty into a holding tank, but requires an external tank or bucket.

Step 9: Enjoy the Creation!

This tiny cabin is towable with a Ford Ranger. I can even pick up the cabin by the trailer tongue by my own strength and swivel it around if needed. It isn't easy to do that, but I have done it. It is a perfect fixture out on my private wooded acreage, and it is one of my favorite accomplishments. (For some reason a few of the photos loaded upside-down, and I apologize.)