A trench coat is a raincoat that typically hangs down to below the knees. In addition to buttons, many trench coats have a sash that can be tied to help keep the coat closed against the wind. Many drawings that feature a trench coat often depict the trench coat slightly open, revealing the body of the person wearing it. This isn't a difficult process and can be accomplished with a few simple alterations to the trench coat design.
Draw the coat on your subject, following the lines of your subject's body to create the arms and body section of the trench coat. Draw all of the coat except for one side (the side of the coat you'll alter to create the open look).
Draw the sash tie of the coat hanging limp on both sides of the coat. Create each sash by drawing two slightly wavy vertical lines running side by side. Connect the two lines at the bottom to complete each sash.
Draw the side of the trench coat that will appear open by creating a vertical line extending away from the subject wearing the coat. Draw it about the length of your subject's arm, then ring the line toward the bottom edge of your drawing surface at a 45-degree angle. This will be the open flap of the trench coat, and the surface here is the interior of the coat.
Curve the inner and bottom edges of the trench coat (the side that isn't drawn as a flap) slightly away from the subject of your drawing to create the effect of the coat falling open on this side as well.