Tailoring shirts is certainly not an easy craft, but if your shirt fits you well around the shoulders, it's very easy to adapt the waist hem to your body.
First, practice with an old shirt. If you get a good fit once, you can repeat the same process with any new shirt.
The easiest is the pinch method (see it in action here):
- Put your shirt on, but inside out. The inner seams must be on the outside.
- Pinch the fabric all along the side seams together until you get a good fit. It must not be skin thight, but still close to the body.
- If the sleeves are too baggy, you can pinch the underside of the sleeve as well, but you must keep them wider than skin tight, otherwise you won't have room to move your shoulders or ellbow.
- Secure the fabric with safety pins or straight pins along the line where your fingers pinch it.
- Carefully take off your shirt and lay it flat (still inside out).
- Sew along the line you marked with the needles, either up to shortly before the armhole (if the sleeves are ok) or all the way to the wrist (if your sleeves are too baggy).
- If the amount of fabric you took in is small (less than an inch), you can probably leave the seam as it is. If you took in a lot of fabric, the excess will bunch up and show on the outside. The video I linked above shows how to trim and clean the seams.
Sewing straight lines is the easiest thing you can do on a sewing machine. You align the fabric, push the pedal and the machine does the rest for you. Once you finished sewing, Lay the new side seam flat on an ironing board and press the excess fabric towards the back of the shirt to make the lines look crisp.
If you don't have a sewing machine, do not use fabric glue! That will ruin your shirts with stains. Instead, there are some tools you can use, like ZipSeam, which you can see in action in this video.
If you adapted the side seams, but now the fabric is bunching up in the back, you'll need to add small back darts. If you found the adaption of the side seams easy enough, adding darts is no more complicated than that.