Upload order matters
Instagram shows your newest post first, so a grid photo has to be uploaded in reverse - bottom-right tile first, top-left tile last - for the finished grid to read correctly. Every tile this tool produces is already numbered in that upload order.
The tiles need to post consecutively with nothing else in between, or the grid will be broken up by unrelated posts in your feed.
Frame it before you split
Drag to reposition and use the scale slider to zoom - the grid lines update live so you can see exactly what will be cut. Nothing is generated until you click split, so take your time framing the shot. Everything happens in your browser; nothing is uploaded.