The link for submitting "Comments and Corrections" is on the right side of the page under "Page Tools." Makes sense. From there you can "Add an Alternate Name" and a brief note about why you are making the correction. I've been surprised at how quickly staff (or an automated system?) is approving my submissions; most are taking just a few days to show up on the website.
Note: If ten people in a household are all indexed under an incorrect surname because the head of household's name was recorded or indexed incorrectly, you will have to submit a correction for each individual. Otherwise, Ancestry will not apply a correction for one person to the entire household.
You probably need to have an Ancestry account and be logged in for these links to work, but here are some examples of my handiwork:
- Hartwig Heinrich VOSSIEK: The family's surname was recorded as FASSOCK, which makes sense if you know a little about German and what to expect from enumerators. But to a newbie, it might take a while to find this family. I know from experience. (Worse yet, daughter Johanna Friederike Louisa VOSSIEK was counted as a boy named Frederick.)
- Hermann Conrad Heinrich POHL: Recorded as "Henry BOUL," this wasn't easy to find. After learning about how German sounded, I searched for a family that may have been recorded as BOHL. Still nothing. I don't even remember how I found this record, though it was probably by filtering results based on given name, age and nativity. If the occupation was searchable, this would have been fairly easy. Henry came from a long line of tanners.)
- Rocceanna (KENSLER) HEATON-SPENCE: The family's name was misread by an indexer and was showing up as "SPIECE." Additionally, the spelling "Rocceanna" was used in several records, but so was "Roxanna" or "Roxianna." I added a note about that, too.
In all, I probably submitted several dozen corrections or name variations to make individuals more easily found. None of these are recent finds. After noticing the correction tool, I took a quick glance at some of the census records I've transcribed (I transcribe all of them I find) and submitted my notes. It's a really easy process and won't take more than a minute or two per census record, depending on the size of the household. To steal a phrase, just do it.
PS - This is not limited to Ancestry's census collection. Corrections can be submitted for records in other databases.