No problem Greg, the key thing (for an easy time with this) is to use the ELM327.
It has an AT command set (remember that from the dialup days ? )
You would need to only start the bluetooth connection, then send it a couple of AT commands which tell it which OBD protocol to use, EG 'ATP5' for protocol 5, and a couple of simple CANbus initialisation commands (but even they are sometimes optional) and it's ready to talk to you
You now know almost everything that you need to know....
There are just two other things yoiu need to know exist - a 'mode', and a 'parameter ID (PID)'
Mode '01' (in ascii) is a read reaquest. So the start of your message begins '01xx' and yoiu replace 'xx' with the parameter ID which you want to read, for example RPM has a standard parameter ID of '0C'
So you've made the bluetooth connection and have a terminal open on your Android/Windows device or whatever.....you send a couple of AT commands to initialise the ELM (the ELM327 will usually store those for the next session too so yoiu might only ever need to enter them one time).....and then if you want to know the current RPM then you really honestly and truthfully only need to transmit '01OC' from your terminal.
For 010C you will get a response which will include the RPM of the engine. The response will need a little bit of decoding, but it is nothing more difficult usually than a coup;le of hex bytes.
So to finsh the job...
for coolant temperature send '0105'
for oil temp send '015C'
Using the list o f PIDs from here, http://en.wikipedia.org/wiki/OBD-II_PIDs
,you can see that you only need to send the first couple of bytes to the ELM327 and it will do all the hard work and give you a meaningful response....
There are a couple of caveats. The first is that there is usually a limit to the rate at which you can request parameters. Asking for a 20Hz refresh rate will cause problems (there will be no ill effects, but the ECU will respond with a 'request refused' message). The other problem is that some manufacturers mask off some PIDs and you need to be in a factory diagnostic mode to be able to read those....hence how they charge so much for diagnostic equipment sometimes....
There is enough info on the web to make the trickier PIDs simple to read so I would have a go anyway.
You can save some money too,because if you connect to your bluetooth dongle and you have an EML light on your dashboard showing a fault, which you need to clear, then you simply send ascii '04' to the ELM and it clears the ECU warning light !
Other things you can do are more interesting. For example my car has had a problem with the diesel particulate filter in the exhaust system which blocked and caused me problems. To reset the soot counters for it (and to tell the car I have a new filter) I needed to only go into the test mode '06' and issue command '15', and now I have control of when the car regenerates the filter. It normally regenerates a new filter 15 hours after the test has been run. I drilled a hole through my filter to make it free flowing and have piggy-backed a microchip pic in the dongle, where the old bluetooth module was and now it resets the filter every 12 hours, so it never trieds to regenerate an empty filter !