Tech editing is basically going through the pattern with a fine-toothed comb, while following your Style Sheet. This ensures there are no problems with the pattern, and that you can still develop them in your own style. And for any new designers who have never developed a Style Sheet, I can help you with that as well.
What I Check For:
Well, to be fair everything, including but not limited to:
Ensuring that all materials listed and correctly described.
Ensuring that gauge, number of cast on stitches, final measurements and schematics are present and correct.
That Stitch counts after increases and decreases are correct and fit available rows (rounds).
That Stitch pattern(s) are correct on all rows (rounds), and are balanced in cast on numbers.
Consistency of grammar, spelling, and arrangement.
Check definition of abbreviations, and consistency of their use.
Ensure the math in the written directions is correct.
Ensure all the steps are included, and are laid out in a logical fashion.
Ensure that it conforms to Style Sheet / Pattern Template.
Check that the chart(s) match the stitch pattern(s) and written directions.
Ensure that all hyperlinks work, and that tutorials match the pattern description.
Check header and footer for the pattern title, page numbers, and copyright.