I have been taught to write them in the past tense, but I have read somewhere that you shouldn't. Any input is appreciated.
I think it might depend. In the method, use the present tense like pour or mix. Don't use -ing words though cause I don't think the examiners will like that.
In your plan or if there's a part where you have to say what you're going to do in the experiment like "One way in which the presence of the ions can be tested could be by doing this...", use the future tense. If it's your conclusion or reports on what you have done, use the past.
I'd suggest talking to your teacher about it or looking up exemplary investigations and it might give you some idea.