The first thing you will need to do is grease the inside of the headtube where the bearing is seated. You need to replace the lower bearing first, making sure that it is positioned so that it fits correctly into the frame. The fork then needs to be inserted into the headtube of the bike. The top bearing can then be greased and replaced. Once again, you need to ensure that it is positioned correctly into the frame. The compression rings and seals can then be replaced. Then the stem can be replaced. You can then replace the top cap, making sure that it is tightened correctly and takes up any slack. Be sure though to not overtighten this, as this is easily done. If you do, you can damage the bearings