Lets start with the forms.
If you're buying a bike, you'll need some forms from here: www.dot.state.tx.us/forms/vehicle_titles.htm
If you already have your bike, you can skip the forms pertaining to title transfer...You'll need a copy of your title or MSO to get your bike on the road, though. If you didn't get a title with your bike, don't worry, you can still get a title. Yes, even if the bike changed hands several times without a title. More on that in "What if I don't have a title?"
Forms that you will/may need are:
VTR-34
VTR-271 (If the seller has lost the title**)
130-U (filled out by you and the seller)
VI-30A (but don't bother looking for this one, you have to get it from the place that inspects your bike)
Out of State Inspection Form (I needed this to get my plate and tags. They gave me the VI-30A back)
Here in Texas, you need a Headlight with Hi and Low beam, a left side mirror, a horn (get one that will get the attention of other motorists, not a squeaker), a taillight with brake light, rear reflector (I used reflective tape) and DOT approved tires. Of course, you will also need insurance***Be sure to let your insurer know what you're doing to make sure you have the correct policy***.
There are several companies that sell parts to make dirt bikes street legal, often in complete kits. These kits are called Dual Sport Kits, or Enduro/Dual Sport Lighting Kits. Parts are also sold "a-la-carte" so you can piece together your own kit.
Baja Designs and Tusk are probably the most well-known kit builders. I found many parts, for almost all the kits for sale, to be made by K&S Tech, just marketed via different resellers/kit builders.
If your bike has lights already, you don't really need a kit, just some inexpensive parts to make your bike legal.
The next step is getting your bike inspected. Just take it to any State Inspection Station, just like you would an automobile.
While you're getting your inspection, be sure to have them fill out a "Green Sheet" AND an Out of State Inspection Form. The shop should know exactly what you're talking about when you say either "Green Sheet" or form VI-30A. They may question the Out of State form, but hopefully you can get them to fill it out. If not, just tell them what you're doing and that you need to show proof that the bike passed inspection to get tags.
You may have to take your bike to more than one State Inspection Station. The first station I took mine to told me that "you might be street legal with no blinkers, but you can't pass inspection". The shop 4 blocks away passed my bike without a problem and was just as confused as I was about the previous inspector's comment.
Even after passing inspection, you could run into other issues as well. I had 2 Texas State Troopers, who are also Motorcycle Driving Test "testers", tell me that any motorcycle manufaturered after the 1970s MUST have turn signals. This is absolutely NOT true. I was told by the second officer to contact the Motor Vehicle Inspection Divistion of txDOT, so I did. The MVI agreed that turn signals are NOT required. Turn signals are not part of the inpection items on a Texas Motor Driven Cycle Safety Inpection and are therefore NOT required in Texas. They referrred me to this site for verification: http://www.txdps.state.tx.us/vi/inspection/veh_class_list.asp?class=Motorcycle+and+Motor-Driven+Cycle&classsubmit=Go
Now that you're inspected and have all your forms, go to your local Tax Office and give them your forms. You may have to help them get this done. Be nice and go in with as much information as possible. This isn't something they deal with regularly, and the person you get may have never done this at all. Any information you can give them will help.
If you don't have your title, you have a couple more steps to go, and more $$ to spend. See "What If I Don't Have a Title".
So, quick review:
1. Bike with clean title, unless you need to apply for a title bond.
2. Insurance
3. Install all needed parts to make your bike road legal.
4. Inpsection, get forms from inspector, (then get your title bond, if needed)
5. Go get your new title and plates!
**See "What if I don't have a title?"