OK look at the main bit, L shaped bit the antenna screws on to. At the opposite end of the main tubular shape to the end the antenna screws onto there is a cap you need to remove and in the middle once you've removed the cap you'll see the centre pin. To prepare the coax you strip back about a third of an inch or so of the outer braid and insulation, folding some of the braid back over the outer insulation, leaving the 1/3 inch of insulated centre of the RG58 protruding. Strip just a little of that off and tin the centre wire. Feed it into the tube coming out the side and solder the centre of the coax to the centre pin. Replace the cap.