Create a new file from File->New with dimensions you need. Turn on the View->Ruler. Select Shape tool->Rectangle Shape tool. Draw the rectangle as per your need on four sides. You can customize the color you wish to fill in rectangle shape from options below.
Once done you can either merge all these shape layers after Ctrl Selecting them and right click->merge layers or keep them as PSD for future editing. You can also save as JPEG and keep the original PSD intact to edit in future.
Hope it helps.