How do you go about getting guardianship over a parent?

You first require a diagnosis and a doctors evaluatio (neuro-psyc) of incompetency in making his own decisions. Without that you should not attempt to act. You would then need an elder law attorney to file papers and there would be court appointed attorney to fight this action should your father wish to do so. A court is loathe to remove a citizen's right to their own person, placement and funds without a proof of complete incompetency. Should your father fight your attempt then you will be looking at 10,000.00 to fight such a case.
You should also be aware that being appointed as guardian is Fiduciary responsiblity and you are legally responsible to have proof of every single penny into and out of your father's assets. If your father fights your actions this is a miserable thing to undertake.

If you're not familiar with the courts and the level of court that could grant guardianship in your sister's and father's area, I would ask your sister to contact the local courts, probably a probate court, and just frankly ask if they have any literature on guardianship, and how to obtain it.

You can also search online for guardianship proceeding information for your father's court jurisdictional area. 

I did a quick search on "Katy, Texas, guardianship proceedures". Try that, read over the articles, then contact the court for issues that might need clarification.

It may be suggested that you hire an attorney to handle the guardianship.   If so, search on "guardianships" for law firms that handle them.  You could also search on "elder law", but guardianships aren't limited to aging issues.  

You could also contact the local county offices, or state offices, to ask if they have any brochures or guidance on getting guardianships.   In fact, thinking over the issue, I think the county might have some resources to help both you and your sister as well as your father.   If he won't allow people in his residence, and is nearly blind, I'm guessing that there are some broader health issues, and/or he may need to stay elsewhere until  his residence is livable.

If you have trouble getting this far, and don't understand something, just post back.