Step 1. Boil the water.
Step 2. Pour some water in the thermos. How much depends on how hot you want your beverage or how quickly you want it. If you’ve got time, you can put just a few inches of water in, secure the cap, and the steam will heat up the rest of the thermos within a few minutes. I let it steam while I’m packing the rest of my food/snacks and getting the last minute things packed up. This is the "green" method because it uses less water. If you need your hot drink ASAP, fill the whole thermos with boiling water and the water will heat the thermos in just a minute or two.
Step 3. When your thermos is hot, or after the allotted time indicated above, pour the preheating water out and fill your thermos with the hot beverage of your choice.