Election officials instruct voters to use a blue or black ink pen.
This ensures your ballot is as clear as possible.
The Santa Clara County Registrar says:
“Your signature must match the signature on your voter registration card... If your signature is missing or does not match your voter registration card, your ballot cannot be counted.”
Tip: match the signature on your ballot to the signature on your driver's license.
The correct date format is MM/DD/YYYY!
You can contact your county elections official in order to be sent a second vote-by-mail ballot. That being said, this can take time to process. So fill out your ballot early to ensure it gets counted.
That's okay! You can abstain from voting for a candidate or write in a qualified candidate who received sufficient votes in their races primary election. But don't randomly mark up your ballot.
Santa Clara County Register says:
“Your voted ballot must be received by the Registrar of Voters by 8 p.m. on Election Day OR must be postmarked on or before Election Day and received within 17 days of the election.”
If your worried about potential USPS delays, you don't have to mail in your ballot. You can drop off your ballot at an official dropbox near you. You can find your local vote center and dropbox here.
To find out if your vote-by-mail ballot was received by your county elections official or if your provisional ballot was counted, you can go to this page and click your county's link or call the phone number listed the county elections office.
Additional Resources
California: Election Tools, Deadlines, Dates, Rules, and Links - Vote.org
If you're not voting for the first time in California, you don't need to show ID to vote. If you're a first-time voter, you are voting in a federal election, you registered by mail, and didn't include your driver's license or California ID number or the last 4 digits of your Social Security number on your registration, you may be asked to provide ID if you vote in person.
www.vote.org

County Elections Offices
A list of county elections offices and their contact information is listed below. Or you can call the Secretary of State's toll-free Voter Hotline at (800) 345-VOTE (8683) for your county elections official's contact information. Click the county name to be directed to the county elections office's website.
www.sos.ca.gov