How do I match this ceiling texture?
Are you sure it's not wallpaper? (sorry had to confirm).
Otherwise the only thing I've seen like that was done with a textured roller (probably basket weave looking at the image) and thinned down drywall joint compound.

This image came from www.all-wall.com


NOTE: matching any 'wet' pattern exactly is difficult as the exact consistency of the original mud affects the depth of the pattern, as indeed does how hard the original person pressed/moved the roller. I would suggest practicing on some scrap drywall first. Try different consistencies of mud and/or pressure until you are confident you can achieve a reasonable match. Or, you can go over the whole area with the new pattern.