How can I evoke a Goetic demon to chase away an unwanted spirit?
Same as any other evocation really:
Carefully choose the right spirit for the task, do divination and also ask any spirits you trust if this will work out well, then evoke them using the method that makes most sense to you.
Ask them if they’re both willing and able to help you with the task, negotiate what they’d like in exchange so you’re both okay with the deal, issue the task very clearly if they confirm they’re able to perform it, then dismiss them/give them license to depart (this bit depends on your working system).
Finally, if the result comes through, deliver your part of the bargain - if not, summon them again and find out what went wrong, and whether/how it can be fixed.
Micheal the archangel. most spirits fear him to no end. try working with the archangel first.