Measure the elevation difference between the top of the stairs and the point where the stairs will land. This measurement must include the thickness of both the downstairs flooring and the upstairs flooring. Measure the elevation difference from finished downstairs to finished floor upstairs. Decide on the height of the riser (the vertical element of a set of stairs). The height of the riser is the distance from the top face of one tread (the part you stand on) to the top face of the next tread. Ideal riser height is typically around 8 inches. Divide the total elevation gain by the riser height, rounding the dividend to the nearest whole number. Then divide the total elevation gain by this whole number to get the number of risers in the stairs you will build.
Draw a picture of the stairs to show the number of risers (it's easy to forget about a tread if you don't draw them out). This will show you the resulting number of treads that will be in the stairs. Typically the number of treads will equal the number of risers plus one. Now decide on the tread width. The rounded edge of a stair tread is the nosing. About 12 inches with nosing is usually ideal. Don't go less than 9 inches. Multiply the tread width by the number of treads. This will give you the projection of the stairs from the upstairs landing (i.e., the length of the stairs). Make sure you have room for the stairs.
Lay out a stringer with a framing square. Put the riser height on one side of the square and the tread width on the other and scribe the square on a 2x12. Stair stringers form the backbone of a set of stairs. Move your way down the length of the stringer drawing out the treads and risers on the 2x12. Cut out the scribed parts and make several more stringers. Now you're ready to assemble and place your staircase.
Once the stair stringers have been created, cut the notches for the stair treads into the stringers with a hand-held router (Image 1). Each notch has to be cut to a precise size, angle and distance apart so that each stair is spaced properly and level. Cut the individual stair treads to precise dimensions with a mortise-and-tenon design. Once the treads and notches are all cut, individually position the treads in the stringer and tap into place (Image 2).
With all of the treads inserted, put the second stringer into position (Image 1). Clamp the completed assembly (Image 2) securely and carefully move it into position for installation. Carefully lift the staircase assembly into position (Image 3) and secure it from above with straps (Image 4).
Bolt the assembly into position, using 2x4 lengths to brace the stairs securely (Image 1). Bolt the stairs securely, with the metal hardware hidden from view. Install the final stair post (Image 2), and add some joist and cross-member support beneath the floor (Image 3) to provide reinforcement to a weight-bearing post adjacent to the stairs.
First attach the railing loosely so that measurements can be taken for placement of the spindles (Image 1). Once placement for the spindles has been measured and marked, drill out the holes for the spindles -- 34 in all (Image 2). With all the holes drilled in both the bottom and top rail, place the spindles in position in the bottom rail (Image 3). Then install the top railing and secure with screws to complete the stair rail (Image 4).