how to trim shoulder length at home

That is one of the more difficult lengths to trim, since you have to get it really straight.  Since you have wavy hair though, you are lucky since wavy and curly hair doesn't hang in a straight line like straight hair, so if you don't get it exactly straight, no one may notice! 

If you really want to do this yourself, I can give you a couple of tips:
Number one, take your time and go slowly.
2. Cut off less than you really want off, that way if you get it crooked you can take off more to straighten it without having to go shorter than you wanted.
3. You will have to try to hold your hair horizontally between your fingers near the ends (and don't move it from the way it hangs naturally), sections at a time to cut straight across so you take off an equal amount from each section.  One of the most secure ways to hold it is weaved between 3 fingers (the hair held flat - over the index finger, under the middle finger, and over the third finger).  This may be difficult to accomplish at this length in the back, just hold it as straight as you can get it and take your time.
4.  Have someone you trust nearby so that when you're done you can let them look at it and straighten any parts that are not exactly even.  No one who's going to dare to cut off a lot!
5.  Be mentally prepared for if you mess up.  Plenty of people cut their own hair, and there's no doubt you can handle it, but it IS your first time, and that is sort of a difficult length to get at in the back, and we are all a little unsure in the beginning.  If you take your time, it should be okay, but if something happens and you feel you just can't fix it, be prepared to either wear it in a ponytail until it gets longer, or to go to a hairdresser and let them fix it.
Good luck!