To start this process, the first thing you need is the millivolt and sensor reading chart. For us, the chart is reading temperature, but for a pressure sensor it would be PSI, Bar, ATM, etc. From this point, it's really not too hard of a configuration.
After naming your sensor, you want to select the measurement type and the calibration. The measurement type for us is pressure, but it could be a huge range of things including pressure, level, voltage, rotation, G force, etc. Once you have selected the measurement type, you have to pick if you want the sensor to be auto calibrated or none. For our sensors, there will be no calibration because the readings will always be based off the data we enter. For things like a displacement sensor or rotation, you'll have to choose the appropriate calibration.
Now, there are some variations and different ways you can work with the tables. Many sensors, including fuel level sensors, some temperature sensors, and measure resistance, not voltage. On those, they require a "pull up" resistor, but the new RS3 sensor configuration helps you out greatly. You can enter the resistance values and it will tell you what size pull up resistor you need. You can also tell it the pull up resistor size if you want to do that as well!
For folks who have an MXL, Evo4, or other AiM products that use Race Studio 2, check out this great documentation put together by AiM on how to make a custom sensor in RS2.