How to replace chicken with tofu in a curry I've never cooked tofu before and I appreciate that using a store-bought jar of curry sauce and expecting it to work nicely may be a little optimistic.

However, what cooking method should I use to cook the tofu before placing it in the sauce (Korma/Tikka Massala) (presumably not for very long, it'd already be cooked) and serving with rice?

I'd hazard a guess that frying it is the best option but should I just fry it and leave it at that or ought I use some herbs and spices?
If you're going to be adding a curry sauce, there's no need to just throw on some extra herbs and spices.

Use an extra-firm tofu, and make sure you drain/press out any extra water before frying. Wrap your tofu block in paper/kitchen towels, place a plate with some additional weight (not too much, a few pounds will do), and let it sit up to an hour before slicing and frying.