An example recommended process of floor restoration and finishing with water based polyurethan varnish is given below.

Floor Preparation
Clean floors of dust and debris if necessary and inspect for loose or broken planks / boards / parquet blocks. If any damaged boards need replacing or fixing, the attention of the works will start with them.
Pine floor boards are most vulnerable to misplacing due to their long life usage. All nails has to be refit and new one fitted before and floor sanding commences.
Loose solid wood flooring can be fitted with screws, having in mind beforehand that pipes and electricity cables may reside below.
Mark any places which may need filling up / covering after the nailing down or nail replacement.

Sanding the Floors
First flow of sanding is dedicated on removing the current sealing and leveling the floors. This is usually done with 24 grid sanding paper and sometimes may even require 16 grid, for poor conditioned floors.
After the floors are all bear, its time for the gap filling if such is required. gap filling may require sanding the floors another one or twice before getting the perfect solid base for continueing with the finishing process.

Staining the Floors in Prefered Wood Dyes
Once the floors are perfectly finished with a higher grade of sanding paper, we can safely apply the staining. Staining is a crafty work, but with few tries even a novice may achieve a quality acceptable appearance. What's most important within the rules of staining is that - it dries quick, so you have work on planned sections; and it's always applied towards to grain of the wood.

Finishing the Floors
After all required routines have taken place on the bear wood, its time for the sealant to come in place and preserve our work for a long time. The first coat of varnish, should follow up another fine sanding again - this is to insure we achieve a smooth surface of the floors by removing the wood grains we have scattered during the application of the wet varnish.

All is left is apply our next 2 coats of varnish and ejoy the look of our work.