1) why do you need a new programming language? (Just for fun is a good enough reason) Could you use one of the existing ones? What will your language give on top of the others?
2) Define all the special things that this language will do eg switch on motors, change temperatures
3) Define the syntax of the special tasks, variable declarations. Throw in all the things you like from other languages, leave out the stuff you don't like. For instance, Algol 68 had if .. fi, case.. esac, do .. od. But it didn't have function..noitcnuf, program..margorp. I didn't like that but it wasn't my language.