The video that flfishnkayak posted pretty much covers everything you need. I also have a Harbor freight 4x4 trailer that I bought for $140 bucks (on sale) that I keep my TI on.
Here are a few pics of my setup, not sure if you will be able to use any of it though since they are different types of boats (my TI is 20 ft long)
Here is the boat and trailer mounted on the car ready to go.
Here is the back of the Harbor Freight trailer
Here is the front of the trailer
Here is the whole trailer
I've been using my trailer in salt water only for about a yr and a half now every weekend, and the regular steel rusts like you simply cannot imagine. I have never put the trailer into the water (don't think I ever will). I ended up once everything was completed getting some truck bed black spray paint that seems to hold up much better that the factory red paint. I replaced as many of the bolts as I could with stainless bolts and nuts that I got at Lowes (about $20 bucks worth).
My trailer is a two piece trailer that doubles as a launch cart, the front half of the trailer is held on by two clevis pins so if I have to walk the boat across the beach and down to the water I just remove the clevis pins, leaving the front half by the car, then walking the boat strapped down to the back half of the trailer (with the boat slid back onto the balance point of the trailer) then walk the boat down to the water and launch, I just leave the trailer (launch cart) by the beach until I'm done. The front half breaks down and fits into my SUV. We don't have a garage at our Key West house so I break the trailer down and place it in the fenced in compound between our houses along side the pool area next to the TI. I just tilt the trailer upright and carry it thru the gate into the compound then store it against the wall.
It actually works out ok (way better than trying to store the TI on the roof of my SUV for a month at a time, then going out every day). I doubt you will need any of those capabilities.
My original plan was to use the steel frame as long as it lasts, then replace with aluminum as it rusts out. All in all I think I have about a hundred bucks worth of aluminum on there now that I bought at Lowes, if your serious I'm sure you can get the aluminum much cheaper at a metal supply place.
Wiring is very simple and straight forward, you will have to do that no matter what trailer you buy or make.
Pretty much any trailer you buy mail order comes dis-assembled in boxes and must be bolted together and wired. It took me one afternoon do put it all together and get wired up (it's not hard and you don't need any special tools typically).
The 1/2" PVC tubing on 11 inch centers is what the boat rests on, I later replaced that with 1 1/2" pvc tubing, about 3 ft longer to provide better support for the hull. I also added a paint roller built onto L brackets at the back of the trailer to make it easier to roll the boat onto the trailer (paint rollers work nice because they are cheap, and are actually very strong and spin easily on 1 1/2 PVC), If you price out real boat rollers you will realize they are not cheap (my paint roller thingy was about $5 bucks to make).
Warning: With my trailer, the boat itself is part of the structure of the trailer, connecting the front half to the back half because it's a two part combination 'trailer/loading cart' , I'm not recommending you do the same.
If it were me I would just follow what flfishnkayak did, but before using the trailer the first time (after all wiring is done) give the whole works a coat of truck bed liner black spray paint (available at Walmart for about $5 bucks),one or two cans should do it. I also covered any exposed bolts and nuts sticking out the top with roofing rubber (from a caulk gun) to prevent ever scratching the boat (before painting), basically you just ooze it on then cover with 3M masking tape to smooth it out, once setup just remove the masking tape. Another thing I did was bury the leaf spring joints in the same roofing rubber so they don't rattle on the road (takes a couple days to dry), I just jacked the trailer up a little, then filled the open area between the leaf spring and the shackles with the roofing rubber and let it dry a couple days, makes a huge difference on how noisy the trailer is.
I'm not sure it's worth paying a lot more for the bigger tires, I like my trailer very low (easier to load), and if I remember the larger tires were pretty expensive, ( the trailer came with the smaller tires for free). If I ever wear these out I will likely replace with the larger ones, but if the trailer comes with the smaller ones, I wouldn't worry about it too much (why throw away good tires).
Good Luck, hope some of this helps