It's time to stain your hardwood floor when the sanding, buffing, vacuuming, and clean up is completed.
The following instructions deal with oil-based stain, not water-born stain. If you are using water-born stain, you will need to water-pop the floor prior to staining. To water-pop is to slightly dampen the floor in the same manner as you stain the floor, from wall to wall, in 2 - 3 foot sections. When it is dry, then you stain the floor.
Many paint stores sell pigmented stains. It is essential to stir pigment from the bottom of the can thoroughly, mixing well to avoid a multi-color result. You should stir any stain thoroughly, regardless. I like to dump my stain into a larger container to keep it from splashing on the walls and other items in the area I'm working in. In that way, you can combine containers of stain to maintain color consistency on the floor.
Work in rectangular sections of the floor, 2 - 3 feet wide, from wall to wall, in the same direction as the boards run, with the grain. This is essential because otherwise stain irregularities can occur. Don't get ahead in wetting the edges with stain before the middle areas. Use the brush to wet the edges, being careful to avoid trim and walls. Use the pad painter or lambs wool to fill in the middle.
Your goal is to get the entire section wet with stain, and then wipe it dry completely with a cotton rag. Wipe on - wipe off! Keep moving across the room, working your way out the door or exit from the area.
Allow stain to dry at least 4 hours or overnight before walking on, working with, or using the floor any further. Anytime you walk on or otherwise use the floor after staining, extreme care should be used in keeping it clean, dry, and dust-free. Debris or moisture can easily be brought in from the bottom of your shoes, for example, with bad results.