Instagram’s grid is 3:4 now
Instagram’s profile grid shows each post as a 3:4 rectangle, not a square. Tiles cut at 3:4 (1080×1440) fill those spaces exactly, so the picture lines up across posts. The square option is there for the older look.
Post them in the right order
Your newest post sits top-left, so a grid is posted backwards: bottom-right first, top-left last. The tiles are numbered in posting order so you don’t have to work it out.
Post all of them in one sitting. Anything posted in between shifts every tile by one and breaks the picture.