The first part of making any costume is to study and understand the person who you are trying to replicate. The case is no different here, since the first thing which you will have to do, is to try and watch some Dr. Horrible episodes to get a better idea of just what he looked like.
Once you have gone on to watch the episodes, go ahead and buy all the items, which you need to make the costume. Given that he has a very minimalistic costume, this shouldn't prove to be too much of a problem. However, in order to really pull the look off, try and get goggles like the ones he has, since those are a little harder to find. At the same time, you are going to want to get a toy laser gun, which represents the one he has, as opposed to being any other kind.
Once you have the goggles and the gun, you need to go on and colour your hair blonde. You can either dye it, or opt to put in temporary colour. Just as long as it looks blonde you shouldn't have any issues.
Now put on the lab coat and the white boots. After you have done this, place the goggles on your head, while holding the gun in your hand.
Remember to put on the latex gloves as well.
Now look in the mirror and try to spot any problems with your costume. If you think something is off and doesn't match Dr. Horrible then try and fix it at home, if not, then ignore it and just go out and have fun in your costume.