Open the mapper
The sheet column mapper is the tool for this job, and it exists because pasting a whole sheet into an order box is where most of the damage happens. It takes a block of rows, reads the header line, and returns a column map you can inspect before committing anything.
Nothing in the step is destructive. The original file stays where it is and the mapper works on what you paste into it. That matters when a mapping turns out wrong halfway down a long list, because the fix is another paste rather than a repair job.
What to paste in
Paste a header row plus the rows underneath it, copied straight from the sheet. Keep one row per item and one field per column. Merged cells, blank spacer rows and a second title line above the header are the three things that break a clean read.
If the sheet has extra columns, a batch code, a seller comment, a private note, leave them in. They pass through as unmapped columns, which is more useful than deleting information you may want later. What should come out is anything that is not a row of data.
A first pass of twenty or thirty rows is enough to learn the shape of the file. Move the whole sheet only after the small pass maps cleanly, because one mapping error repeats itself on every row below it, and a long paste makes that error harder to spot.
Reading the column map
The map lists your headers against the fields the receiving side uses, and each line reads matched, unmatched or ambiguous. Matched means the header was understood. Unmatched means the column has no home yet. Ambiguous means two headers point at the same field, which is the line to fix first.
Check the address column hardest. A block that points at wrapped platform addresses will not behave the way a block of marketplace addresses behaves, and the difference shows up at submission rather than at paste time. One column, one address type, is the rule that keeps a move predictable.
Field note N03 follows one sheet through the whole move, before, during and after, including how to put things back when the first paste goes wrong. Reading it first is cheaper than reconciling a bad paste afterwards, so do it before pasting anything you would hate to retype.
Common snags
Snag one: a header that means something to you and nothing to the map, such as good ones or the blue one. Rename the header instead of retyping the rows. The map reads headers, and a clear header fixes every row underneath it at once.
Snag two: mixed link types in one column. Wrapped addresses and marketplace addresses sitting together produce a column that half works, and the failure stays silent until submission. Split them into two columns before you paste, and rebuild the wrapped ones first.
Snag three: duplicates nobody noticed. A sheet edited by several people often carries the same item twice with different notes. The map will not flag that, so count your rows before and after the paste and reconcile any difference. When the two counts disagree, the move is not finished.