An attractive floor can be created cheaply by sanding floorboards with a heavy duty sander, and applying a flooring grade varnish. Rugs can be laid where people walk most often.
Sanding a floor is a fairly simple job, but it is hard work and usually creates a lot of dust, so wear a face mask (these can be bought at any DYI shop). Otherwise the dust will irritate lungs and eyes. Sanding can also be extremely noisy, so it is wise to do this at a time when it will not disturb neighbors, and to wear sound protective equipment such as earplugs, which can also be bought in a DYI shop.
The sanding is done with a floor sanding machine which uses abrasive belts, and an edging sander. Sanding can of course be done by hand but a floor is an enormous job and it is not advised that this is attempted by hand. Floor sanders can be hired from a tool company, and hire for a weekend or a couple of days should be sufficient. While the floor can be sanded and edged with machine sanders, it can be finished by hand sanding.
There are various grades of sanding belt. Begin with a coarse grade, then follow by medium, and finally with a fine sander.
On fairly new boards that have not been stained or become too dirty, sending may not be necessary before applying varnish. Get rid of surface dirt by washing the boards with hot water and detergent. Pay particular attention to removing dirt from nail holes. When the boards have dried, ensure that it is clean enough by thorough inspection.
Any damaged boards will have to be replaced, but new boards are unlikely to match exactly the colour of the old. For a pristine job, replace the defective board in a prominent position with an existing board from a less obvious place, and then replace that board with the new one. Ensure that this spot is likely to be hidden by furniture or a rug.
A sealed floor should be swept regularly, as floor dirt can be abrasive and damage the shine of the varnish.