Make the wheels first and set them aside. I used Size 1 Pampers Swaddlers (the amount used will vary depending on the size of the tires, but I would buy at least an 84 pack of diapers to be safe). Obviously, you can use any type of diapers you like, but I was partial to these when Felix was first born. I followed the instructions found in this video for the wheels and they came out perfect. I wound up using the springform pan I have for the larger wheels and a Pyrex container for the smaller wheels. I wrapped the wheels with a couple large rubber bands before removing them from the pan.
I purchased thick green ribbon (2 rolls) and thin yellow ribbon (one roll). I cut the ribbon to the lengths needed based on the size of each of the four wheels. I then affixed the yellow ribbon to the center of the green ribbon using a strong adhesive tape. I attached the completed ribbons around the wheels using double sized tape. I then used two paper towels rolls slid into the middle of each of the wheels to anchor the wheels together. We then put a pair of baby socks into each end of the paper towel rolls as hubcaps (we didn't use any particular color for these).
I used two dishwasher baskets for the tractor itself. I used the Babies R Us brand that Kate had registered for, which worked perfectly because come in two different sizes (one for each part of the tractor itself). However, you could easily use a box of wipes for the larger part of the tractor and purchase a Munchkin dishwasher basket for the smaller part.
I wrapped the smaller dishwasher basket with a hooded towel (it may take several folds to get the size you need while also hiding the hood portion of the towel) and affixed it using double sided tape. I then used the same combination of green and yellow ribbon from above to wrap that part of the tractor.
I wrapped the larger dishwasher basket with two receiving blankets (one to cover the front and back of the basket and one to cover the sides). It may take a few folds to get the right size for the receiving blankets. I affixed each blanket using double sided tape.
The steeples on top of the tractor were made using a pair of baby socks. For the longer steeple, I placed a sharpie inside one of the socks (which can later be used to write your child's name on things for daycare) and wedged it into the ribbon used around the smaller dishwasher basket. The smaller steeple is a rolled sock held together with the thin yellow ribbon. I then used double sided tape to affix the smaller steeple to the top of the small dishwasher basket.
The headlights are two craft puffs (not sure what they're actually called) affixed with double sided tape.
The only pain with the tractor is that I couldn't find a way to attach everything. Since we also bought Matt and Kate a Radio Flyer wagon, I rested the dishwasher baskets on top of the wheels (which have been held together with the paper towel rolls) and placed paper "grass" all around it to keep it from falling apart.
Another tip is that I placed all the extra socks and receiving blankets (since you will likely buy these in packs of 4 or 6) inside the dishwasher baskets.
I hope this helps anyone looking to make this in the future. We were really happy with how it turned out!