DP and I did ours last year. It was worth the result but a lot of work.
The first thing is to make sure the nails are really knocked in- not just flush with the floor but a couple of mil under so that they don't become proud once you have made a couple of passes with the sander.
We hired a floor sander and edging sander, both are essential. Try to hire from somewhere that will offer you use or return on the sanding sheets and then get more than you think you will need. It is a pain to have to stop work just to go back for more. Have the measurement of your room with you when you get the sander and ask the hire store to suggest how many sheets you will need.
Sand across the boards rather than along them. We did several passes with the coarsest sheet, only one with the medium, and then a couple with the fine until it was smooth to touch.
It will get horribly dusty. Open all outside doors and windows and seal up any internal ones with masking tape.
Leave filling gaps in the boards to after you have sanded. To fill gaps between the boards we used foam draft excluder, which I got from Nigel's Eco Store (am on phone now, will do link later). It gives a really nice neat finish to the floor and does not at all look like you have filled the gaps with foam.
We stained the boards using Ronseal floor dye- none of the colours was exactly what we wanted so we mixed two.
We finished the floor with three coats of Ronseal floor oil, which is quite pricy, but has left a super shine, and will not flake like varnish when it wears. It did need three coats though.
Our floor looks lovely now- like fresh shiny conker.