A sliding window is weatherstripped similarly to a double-hung one--just envision the double-hung one on its side. If only one sash moves, nail a spring bronze strip in the side channel that receives the sliding sash. Then nail tubular gaskets along the top, bottom, and side of the sliding sash. For windows where both sashes slide, nail spring bronze strips in both side channels and attach tubular gaskets to the tops and bottoms.