What should I do if I intended to do something in the future and said, "Tomorrow I will do such and such," and forgot to say "in shaa Allah (God willing)," then remembered this omission later?

There is no blame upon someone who does not say "Inshallah" (God willing) when intending to do something in the future, unless they have sworn to do that action. However, it is better and more courteous to say it, as everything is subject to the will of Allah. If one forgets it and remembers it after some time, there is no sin upon them, and they should say it whenever they remember, acting upon the verse: "And remember your Lord when you forget."