I just took the valve stem out and squirted the foam in. I then spun the tire to try and get the foam evenly distributed. I did my best to try and keep the opening open as the foam expanded out the opening. I let it sit overnight, then cleared the opening and pumped a little air in there a few times from time to time, leaving the valve out. After a couple days it seemed to be setup so I put the valve back in and added a little air. It all worked for a while (several outings), but then it started getting lumpy in a few spots if I didn't have enough air it. Eventually I got tired of messing with air filled tires (a big pain) so I went to the Hobie HD foam tires and never looked back, I love the HD tires.
I figured out that it's only 10% of the time that I launch on really soft sand beaches, 90% of the time the hard HD tires work just fine, plus the boat rolls easier on hard surfaces with the HD wheels.
Last year I built a trailer for my TI and have not used the scupper cart since. My trailer is two pieces so I can use it as a trailer or a dolly cart to pull the entire boat down to the beach by hand.
Sometimes when we go down to Key West I leave the trailer at home and car top our TI, we typically launch at Higgs beach down in Key West (soft white sand). What I do is roll the boat all by itself down to the water on the scupper cart, then carry separately the AMA's, and sails then assemble the boat at waters edge. I just find it too difficult to haul a fully loaded TI with a scupper cart (Even with Hobies special cart attachment), it's just too much weight for me to pull (about 200lbs +), the Hull alone is around 100 lbs, 'huge difference'.
I have given up completely trying to haul around a fully loaded TI, except on my trailer which I use 90% of the time.
With the trailer it's especially nice because I don't break anything down on the boat, All the seats, motor, anchor system, sails, AMA's, etc just stay on the boat all the time with the boat stored in the garage (on the trailer). When I'm done sailing I just pull in to shore, fold the tramps (the most time consuming task), drop the jib (using the halyard line), then just lay the jib and spinnaker in the boat with all the rigging still attached. I pull the mainsail down and lay it on the boat. I then just pull the entire boat onto the trailer, I have a winch, and usually use it to winch the boat onto the trailer (just like powerboats do), especially when I'm totally exhausted from sailing all day. I then fold the AMA's in, strap the boat down, and I'm on the road, very fast and efficient. When I get home I just rinse the boat and motor off, and put it all away in the garage, ready for the next time.
When we car top the TI it's a whole nother story, breaking the boat down completely and taking everything off and storing it all in the back of the car, is a royal pain in the butt for me. Then storing everything for the boat in 5 or 6 different places in the garage is a pain, also just the boat alone (without the trailer) takes up an entire parking space in the garage no matter how you store it (it has to be at an angle in our garage, because it's too long).
The best investment I ever made was buying that $140 dollar harbor freight trailer.
Here is the TI on the trailer ready to go out.
Here is the back half of the two piece trailer (that can be used as a beach cart by detaching the two 1" aluminum tubes that just clip to the frame.
Here is the front half of the trailer (which would stay with the car, or break down and stuff into the back of the car.
Here is a shot of the whole trailer. The extra aluminum and fittings cost me another $150 dollars or so, I got everything at Lowes or Home Depot, and it took a weekend of my time to build. I'm sure some of the steel framing will need to be replaced with aluminum once it rusts out (another $100 of aluminum), but it's been going strong for about a year now with no issues (every weekend I'm out with it)
I made my spray skirts so they fold over the bow and hold all the sails and rigging down on the highway, works really well.
The way I figure things, by the time you get a cart big enough to haul a fully rigged TI around, it gets too large to bring along in the boat anyway (like the big Hobie cart that's like a Cat Trax for example), so why not just make my trailer in two pieces and do the same thing (work as both a trailer and a beach cart), a lot cheaper as well. I leave the back half of the trailer at beachside just like they do with the regular Cat Trax trailers for the regular Hobie Cats, theirs usually at least a half dozen of them just sitting on the beach where I typically launch next to Sarasota Sailing Squadron. If I had the big Hobie cart, that's what I would be doing anyway.
Hope this gives you some ideas