Where is the best place to start spring cleaning?

Work room by room. Here are some checklists to keep you on track.

https://www.marthastewart.com/267295/spring-cleaning-checklist

https://www.thespruce.com/printable-spring-cleaning-checklists-1900956

I always pick the easiest job first, the one I can get done the fastest. Then at least that place is off the list, eg. hall closet, etc. Then I tackle one room a day, otherwise I would never finish if I just cleaned all day.

Work from top to bottom. Start with vacuuming cobwebs from the ceilings and walls, then down to the trim. Clean your light fixtures next. Put the glass globes in hot vinegar water in the sink. Then get out your bucket of nice smelling sudsy water(I like Mrs. Meyers) and wipe down every flat surface. Use glass cleaner to clean any glass surfaces or mirrors. Vacuum and clean your floors last.

I do one room a day. It is not overwhelming that way. Or you could do one room every couple of days. Save kitchen till last because it needs cleaning and used the most. Make a schedule and stick to it.

Another thought. Get a close friend to help each other. Do a room your house, then hers.