Welcome from Ohio! I know the fear of cutting into beautiful fabric. Three things helped me. First, I purchased some inexpensive material to practice on. Next, I purchased a set of 4 bed risers to put under the legs of my cutting table (I use a card table with a cutting mat on it and it was hard to bend over and cut correctly.) And then I practiced my husband's (former shop teacher) favorite saying: "Measure twice, cut once". It also helped that I purchased a ruler that has a lip that catches on the edge of the cutting mat. Really helps to keep things straight and square.
Hope this helps! You can do it! ;)