A grid post is one picture spread across nine squares of your profile. Done properly it turns the top of a feed into a billboard. Done in a hurry it turns into nine tiles in the wrong order and an afternoon of deleting posts.
The hard parts are the crop, the size and the order, so this preset handles all three. Square crop first, with control over where the square sits in a rectangular photo. Nine tiles at 1080 × 1080 px, which is the size Instagram stops rescaling at. File names numbered in posting order, which is the reverse of reading order, because the newest post takes the top left corner and pushes everything else right and down.
One change to plan around. Instagram now crops profile thumbnails to 3:4 portrait instead of square, so a nine-square grid shows with a slice off the top and bottom of every tile in the grid view. The full square still appears when someone taps a post. In practice that means composing for the middle band: faces, text and anything you cannot afford to lose stays away from the top and bottom edges of each tile.
A grid is also temporary by design. Every post you publish afterwards shifts the whole thing one square along, so plan it as a moment rather than a permanent header.
Nothing is uploaded. The crop and the nine cuts happen on a canvas in this page.