it's hard to explain so here constellationgerard

im gonna just use this random photo

i use 9:16 ( usually it should have it if it's a taller picture but if it's a longer one and you're using a section, crop it to a little smaller than a square and then try it again and 9:16 should show up )
lots of people just use apps to get the perfect size but i like this better because im lazy

phonto

click "move"

and then click both of the buttons on the left so it'll center it both top to bottom and side to side

usually i add a shadow behind it in "style" but that's up to you

then you just save it and usually when you put it in as a cover it only cuts off even amounts of the top and bottom so your text is still centered