I doubt that throttle body cleaning will help your gas mileage...
If you want to do it, with a rag soaked in carb cleaner, open the throttle plate, clean the edges of plate, and the mating sides of the bore.......you do this with the key out of the ignition...
Now remove the negative bat cable....then the pos cable....now hold the two cables together, for at least a few minutes(I use a jumper cable and attach it to both ends of the cables....this resets every module in the vehicle)....
Then, reattach the positive cable first, and then the negative cable.....a couple drive cycles and you are good to go.....
When you say gas mileage has declined, is this based on readings from the DIC? What are the differences in the readings?
Have your habits changed? By that, I mean, do you do more local driving now than highway?