All that you need to do now is put the new pickup in. to do this you may need to make new holes. once this is done you need to screw the pickup in the same way that you took the old one out, making sure that you remember to put the springs back in.
Now that the pick up is in you need to thread the earth wire back through the body of the guitar until it comes out the other side. now you can screw the scratch plate back on and turn the guitar over. You need to melt the existing solder and reattach the earth wire. You may need to add more solder to make sure it holds.
Now your guitar with the new pickup is ready to play. Before you try it you should take your guitar to your nearest guitar store and make sure that it is all ok. This should be free as they should just have to have a quick look.
Now try it out and have fun. You may need to raise the other pickups to try and get the volume right. Once this is done you are completely ready to rock.
Happy Playing! :)