I recently did my first pant alteration, and it was much simpler than I thought!
First thing I did was put the pants on inside out and pinned the sides on the side seam so they would fit to my body.
Then I sewed what was essentially a dart on each side where I had pinned. Make sure that you have any waist band seems matched and that you stay on those side seams so the alterations look natural.
And that’s it! It really is that simple!