Long hair dog grooming - how to do a hygiene cut at home using scissors?

I have a Shih Tzu, recently adopted from a rescue. When we got her she was shaved completely because of mats and such. Now she has been getting a healthy food, and her hair is growing out. Her hair around her butt is really long and shaggy, and I think I need to trim it because she has pooed on it before and I had to give her a bath. I hate to take her to the groomers because I just had her nails trimmed and dremeled a week and 1/2 ago, and her hair is not so long that it needs to be cut (I plan to grow her hair out long anyway). She has a 2 inch top knot at the moment and what looks like a puppy cut, and is very fluffy. The groomers wants 45 to cut/bath/etc her but all I want is the hygiene cut.. but they don't do just that, so I have to get it all done or nothing. There aren't many groomers where I live so I don't have much choice to either cut it myself or pay $45. How do I do a hygiene cut at home? Just trim close to the legs and butt, or just cut it all an inch or what?