Use go in when someone enters a place, usually a building. She went in and sat down. If you're telling a reader the way to do it, that's a how-to request about entering, not a grammar question.
Situations where 'go in' fits
When you're giving a visitor directions, use go in for movement into the building. Please go in through the main entrance. The condition is a door or entrance the person can actually use.
When you're narrating what someone did, go in works on its own. She went in and sat down. No object is needed because the place is already clear from the situation.
When you're describing a routine at a workplace, go in fits too. I go in early on Mondays. It communicates entering the office, and the building is understood without naming it.
Contrast: when nothing is entering anywhere, don't reach for go in. If the fit is the issue, as with The key won't go in, that's a different sense, not movement into a building.
When you're unsure whether to add a phrase after it, ask if the listener needs the route. If they do, add it: Go in by the side door. If not, plain Let's go in is enough.
When 'go in' needs something after it
Go in can stand alone, but only when your listener already knows the place. If you're both standing at the door, plain wording works: Let's go in.
If the entrance isn't obvious, add a phrase telling them where. Go in through the reception area. Without that detail, your sentence leaves the listener guessing which door you mean.
So the decision is simple: does your listener need the route? Name it when they do, and keep go in bare when the place is shared knowledge already.