Here's a step-by-step guide. I've done this for a long time and I can guarantee it's fool-proof.
1. I'm assuming this is via campsite/other town/spotpass. If it is, after you invite her in time travel about three-four days forward. It doesn't matter the time, but it could cause problems if it's before 6 am when you start this. Save after you invite her in and she says yes, obviously. It's important, though.
2. Do NOT log in as your mayor/preset villager first. FIRST, create a new save file. If her plot is there, great! If it isn't, time travel a day or two forward. If it's STILL not there either a) you're not looking close enough or b) you have to go back to the day you invited her in, go in as mayor the next day and then save. However this has never happened to me. Just in case.
3. After that, just keep logging is as a new save file. Do not save your new villager or log is as a mayor/preset until she is in the place you want her to be in or else it will be too late.