I know I have to ask teachers through an email asking if they could write me a letter then invite them to common app, but when do I invite them? Do I do it at the same time I send the email asking them to write a letter or do I do it later?
I wouldn’t invite them until they’ve agreed to write you a letter. Once they have, invite them ASAP so you can submit your applications (you don’t need them to be submitted to apply) and they can start writing. Make sure to send them a Thank-You note after they’ve submitted their letters! 
When I email them asking to write a letter do I ask for them to let me know their decision?
@hannah1212 Asking them if you can write a letter is asking them for a decision. They’ll usually respond with a “Yes, I’d be happy to;” or “Unfortunately, I’ve already committed to other letters.”
Fwiw, I would suggest asking a teacher for a recommendation in person, if possible.
If you’re in remote learning, there may not be a choice.