Gather images of various staircases to research different styles of stair treads. Go on the Internet to search for images or look in architecture or home-style magazines. Pay close attention to how stair treads are designed, so that you can see how such a practical feature can be so different in design.
Look also towards other sources for design ideas. For instance, gather material to create textural patterns by doing a series of rubbings of curious textures around you. Do this by taking a piece of paper, place it over an interesting texture, and with a pencil or piece of charcoal, rub over the paper to reveal a textured pattern. Put them aside for later.
Measure the length, width and depth dimensions of the old staircase treads with a tape measure. Note the measurements down on to a piece of paper, next to quick diagrams of each stair tread up the staircase. Depending on the staircase structure, not all stair treads will share the same dimensions.
Open your sketchbook. Draw the shape of each stair tread on each run of the staircase, using a pencil. This is the basic design shape to start designing your stair treads. Try changing the contour of the open exposed edge of the stair tread, if applicable. For instance, you could create subtle curves to be carved into the contour edge or to specify an alternative edging like metal or rubber grips etc...
Design the stair treads. Decide whether the stair tread is to be plain, patterned, tiled or painted. Consider what materials -- such as wood or metal -- to use to reshape or replace a worn stair tread. Start drawing some possible designs. Refer to your texture rubbings to see if there are design possibilities to create a pattern for the stair tread. Indicate with pencil notes next to each design whether the pattern design is to be painted onto existing wood or carved into new wood. Alternatively a mosaic design of small tiles could be used very effectively to cover existing stair treads. Be experimental and play around with various design ideas.
Apply color to your stair tread designs, using art marker pens or colored pencils to complete your designs.
Choose a design you like, so that you can start making new or refurbished stair treads.