The idler arm transfers steering motion to the right side (passenger side in USA) tie rod which transfers it to the right wheel. The idler arm is held in place by a large bolt for which you have a replacement. You'll need to remove the nut, remove the heat shield over the top of the bolt, push the bolt out (noting how all the washers and dust caps are aligned), pry and push the old bushings out of the journal (use the old bolt to beat the top bushing out), reassemble all the parts in the correct order. The bushing may have to be pulled into place using the old bolt and nut and maybe some spacers. Be sure to lube the rubber on the bushings with some silicone grease to facilitate their sliding into the journal. The torque value for the new nut is pretty high. Don't have it in front of me. Perhaps someone else can fill in this data. It'll take must all the force you can generate on a 12 inch wrench laying under the car.
1984 300D Euro