Hagala of a pot must be performed in a large kli into which the entire pot can fit. It is not sufficient to boil water inside the pot, because it is likely that during the course of the year some dishes sprayed out or boiled over the sides of the pot, in which case the flavor of ḥametz was absorbed into and adhered to the pot’s upper lip – and the lip is not kashered by boiling water inside the pot.
If there is no large kli in which hagala can be performed, one should fill the pot that one wishes to kasher with water and bring it to a boil, and at the same time, boil water in a small kli. When the water in the pot boils, insert the small kli into the boiling water in the pot, which will cause a lot of water to spill over every side of the pot, thus kashering its lip. (See Avoda Zara 76a, and see SA 452:6.)
When it is possible to remove the handles of the pots, some are scrupulous to remove them and clean them. Alternatively, one may clean around them with plenty of soap, until it is clear that any flavor that may have been in the crevices has been befouled. Then one performs hagala (above, section 10). Pots that have a metal fold along the rim do not require any special treatment.
Regarding hagala of a large pot that is kashered by boiling water in it: It can be suggested that one should cause the water to overflow by pouring boiling water into it from an electric kettle. However, the solution I suggested, using a small kli with boiling water, is preferable, because when pouring the water, it cools down slightly, whereas the entirety of the small kli is boiling hot and is itself a kli rishon, and so does not cool down.
Kashering a pot by causing it to overflow onto its outer walls is effective for a kli that absorbed ḥametz through overflow, but a kli that is regularly inserted into another kli and absorbed ḥametz flavor there – as in the case of a ladle – absorbed in a kli rishon, and hagala by the overflow method is not effective. Instead, it must be kashered through complete immersion in boiling water (MB 452:31).