You can use several methods;
1.Hot-air stripper or blow-torch..requires a lot of sanding, & it's not very good around the spindley bits.
2. Nitromors paint stripper..very messy, and it strips bare skin as well.
3.Peel away..This is a paste you apply to the woodwork then you cover it with some paper that's supplied. This causes a chemical reaction between the paste & the paint..very good for intricate mouldings etc. However, it's expensive.
I've stripped many staircases and I use a combination of hot-air stripper & peel away. I use the hot air gun on flat sections, & peelaway on the intricate bits. I would also suggest you get an orbital palm sander & a roll of 80 grit silicon paper. Hope this helps & good luck, you'll need it !