It's pretty easy to take a dogs temp if you have a cooperative dog or someone to help hold him/her.
The reasoning for "how much" ...1" to 3" depends on the size of the dog. I have Bostons, a Frenchie and a Lab. For the smaller dogs, I insert the thermometer just 1" For the Lab, about 3" You don't want to perforate the bowel. Just keep the thermometer parallel to the ground and you should be just fine. It's pretty easy.
Here's a pretty good link complete with pictures.
How to Take a Dog's Temperature (with Pictures) - wikiHow