a/f gauge you will need to tap into/splice into the factory o2 sensor. You can usually tap that line from the ECU, but its not going to work well since it most likely is a narrow band o2 sensor. Oil temp gauge you can use a oil block adaptor like the one greddy makes. The adaptor mounts to where the oil filter is and has tapped holes for the sensor to thread into. I think the water temp gauge you have is the most beneficial since all we have is a stinkin light on the dash. You can get something like the greddy adaptor which you have to mount on the radiator hose. You'll need to cut the upper hose, place the sensor mount between both cut pieces and clamp both ends. The sensor will then thread into the top of the mount. Now the ways i mentioned are the easiest to do without permanently mounting the sensors to the engine/radiator.
Get a quality Scion xB repair manual. Bently brand is the best, Haynes will get you enough info usually. These manuals will tell you where the sensors and sending units you need are.
Locate your coolant temp sensor, most cars have it located on the radiator or near where the upper radiator hose connects to the head. Depends on manufacurer of car. Your repair manual will tell you what wire does what on that sensor, then hook up your guage accordingly.
The air fuel guage needs to read your O2 sensor. You won't get an accurate reading from either sensor because they are not wide-band O2 sensors. Anyways, find your O2 sensor, it's on your exhaust manifold before your catalytic converter. Look at your repair manual to see what wire does what and hook up your guage.
Now the oil temp probably does need that adaptor. some cars have an oil temp sending unit, but those cars also have an oil temp guage, so I'd put money on our xB not having one.