You will need a scissors, the iron, sharp pins or a piece of chalk and needle with a thread.
Here's some tips:
Usually embroidered patches are shrink resistant, their edges come prefinished and does not fray.
The best way to attach your patch it is by sewing a straight stitch all the way around the edge of the patch.
If your patch with no edging then you have to trim away excess of the fabric. Be advised, in order to avoid fraying of the patch you have to leave 5-10mm additional space of the fabric. This additional edges will be visiable at the end of the process. After that you should to turn down the edges, then carefully press the patch with the warm iron.
Now your patch is ready for sewing it on.
Go ahead and put your patch on the right place, then pin it(or chalk outline around the patch) and hem the patch on the fabric or garment.
Then check the result in order to make sure that your patch attached in the right way with no faults. Because later you wouldn't be able to change something. Once you are happy with your job start sewing the patch on, using small stitches again around the edge of the all patch.
Slightly spray a little water on the patch and iron it one more time. Finally ensure that the fabric under the patch didn't wrinkle, also your patch is located on the right place and good looking.