First pick out the paint color...say it's yellow.  Save half the original color and then  mix a paint color that would be a shadow....this is best done by mixing the yellow with the opposite color on the color wheel...in the case of yellow it would be a purple ( not a blue)  If this is confusing find a local artist or a good paint store employee and they'll help you out.  Preferably mix this shadow color in a flat paint...like a primer would be perfect.  Paint all of the walls with this color and let it dry. Carefully measure and draw the vertical lines that will be the bead detail and the seams of the wainscotting....old fashioned ( early 1900's would be a seam every four inches and a bead between each seam) or modern ( a seam every 5-1/2" and a bead between each seam)  It would be helpful to actually look at some wainscotting. It is important that these lines are perfectly straight and true perpendicular to a level horizon.....nothing looks worse than slanted lines.  Now using thin masking tape cover up all of the lines that will be shadows.....for example each side of the bead and one thin line at the seam.  If you have trouble finding very thin masking tape you can take a wide roll of duct tape and tear it off in quite thin pieces...just don't stick the duct tape down too hard or you will never get it off.  This will seem to take forever but it is faster and more accurate than trying to paint a straight vertical line.  I have sometimes used fat string or thin ribbon in place of tape but it is not as accurate...more suited for masonry grout lines than beaded boards.  When all of the masking is finished take the original paint and roll it in a thin coat over everything.  This is where you can get a little creative and play with textures or leave a little more shadow showing through at the bottom (did you remember to draw and mask a baseboard and door frames etc.?)  Peel the tape and it will look great.