It's extremely simple. You can use mumbles overlay to use custom crosshairs, or you can simply convert a crosshair image into a DDS file and name it ui_gadgets.dds and put it inside your textures folder which replaces your normal crosshair.
However, if it's a DDS replacement then keep in mind how the crosshairs move, it can't be a crosshair that doesn't work in the same manner unless you plan on editing a lot more. So if you want something simple then a mumble overlay is the best.