I don't know about your specific header, but I've installed several sets over the years. Usually the installation instructions which come with the headers will usually get you 80% there, but you have to be a little creative, yourself. I usually had to jack the car a couple feet in the air. Sometimes they go in easier from the top, but usually from the bottom. The starter will probably need to be disconnected and removed. The center link will probably need to come out, also. The oil filter may need to be removed, or in the case of rt angle adapter, the adapter may need to be loosened and possibly rotated. It may help to remove the washer & nut from the underside of a motor mount, and using a floor jack and a 2x4 on the edge of the pan rail, lift the side of engine by pivoting the engine over a little and buy you extra room. I think it is usually easier to install passenger side first, then the drivers side.
Sometimes it helps to rotate the headers nearly vertical to get them in and then rotate them back level. It's a lot of trial and error. Sometimes a hammer may be needed, but once you do that, you can't return them, obviously.
