You can find these kits at hardware stores and plumbing supply retailers. A kit should include sheets of insulating foam, adhesive, an adhesive application tool, and instructions. Alternatively, you can create your own tank insulation kit by buying a standard size yoga mat and a tube of high-quality waterproof adhesive. Unless you find a good deal on the yoga mat, however, the price difference may not be that great. Turn off the water supply by turning the oval valve at the end of the supply line clockwise. Remove the lid of the toilet tank and set it aside. Flush the toilet to drain nearly all the water from the toilet tank. For now, leave the small amount of water that remains in the tank. It will help you clean out the tank. Use your preferred spray-based cleaner and a handled scrub brush. Scrub away as much of the rust-colored buildup on the tank's interior sides and bottom as you can. Dip your brush into the water in the bottom of the tank occasionally to rinse it off. Cleaning off the walls will make the adhesive for the insulation hold better. Once you've cleaned off most of the buildup, rinse down the interior walls with clean water, then flush again to empty away most of this water. Use a turkey baster to suck up the remaining water in the tank, or use a sponge to sop it up. Your goal is to remove all standing water from the tank. Your fastest option is to wave a hair dryer on high power back and forth throughout the tank for 10-15 minutes. Or, you can try one of the following:  Position a space heater so that it is blowing hot, dry air into the tank. Check it at least every half-hour and leave it running until the porcelain is dry to the touch. Clip a work light with a 100 watt incandescent bulb to the top edge of the tank. Position it so the bulb is inside the tank. The heat of the bulb will dry the tank in roughly 12 hours. A toilet tank insulation kit comes with sheets of flexible styrofoam that you can cut to fit. Estimate the size and shape of the bottom and sides of your tank interior, and cut the pieces as needed.  Follow the same process if you're using a yoga mat instead of an insulation kit.  Always estimate on the larger side first, then cut the pieces smaller as needed. There should be extra sheets if you make a mistake. You'll have to cut around some obstructions within the tank, like the flush valve and the flapper. If you're using a kit, apply the included adhesive to the back of each sheet of cut insulation with the included wooden stick (basically, a popsicle stick). Try to spread a fairly even layer over the entire surface. Then, press each sheet in place against the tank interior. If you're using a cut-up yoga mat as your insulation, choose a high-quality, waterproof glue or sealant as your adhesive. Apply it according to the product instructions. This gives the adhesive time to fully set. After waiting, just turn the water supply valve counterclockwise to fill the tank back up. Then put the lid back on and you're all set! Insulating the interior should make a noticeable reduction in the amount of condensation on the outside of the tank.

Summary:
Buy a toilet tank insulation kit, or make your own. Empty most of the water from the toilet tank. Clean off any buildup on the walls of the tank. Rinse and drain the tank. Remove all surface moisture from the tank's porcelain interior. Cut the sheets of insulation to fit the tank's bottom and sides. Glue the insulation to the bottom and sides of the tank. Wait 8-12 hours before refilling the tank.