Measure the distance between the two posts you are building your ornamental railings to fit between. Divide this dimension by two. Measure and mark the center of the rail pieces. Measure out from this center mark along the rail pieces and mark each end at half the distance between posts, so that the end spindles are even.
Cut the rail pieces on a miter saw on the two end cuts so that they are now the exact length of the space between posts. Measure the height of the post to the point that your railing's top will be. Typically this is a minimum of 36 inches. Measure the depth of the mortises on the inside of the rail sections, there will be one mortise, or square pocket, cut for each spindle end on the inside of the railing.
Measure and mark your spindles to the height you want your railing, plus twice the thickness of the railing, minus 4 inches and the depth of both mortises (one top and one bottom). Cut enough spindles to fit one in each mortise on the inside of the rail sections, using your miter saw.
Lay the two rail sections parallel to each other on a level work surface and fit one spindle in each mortise, so that the top and bottom ends of each spindle are fitted into a mortise. Nail through the railings into the spindle ends with 2 1/2 inch finish nails, two nails in each spindle.