The amount you earn from a signing is where you start-
1. you deduct the number of signings times your state's allowance for each signature. That fee is Exempt from IRS.
2. You deduct you mileage getting to and from the signing and turning it into the carrier for a receipt.
3. Deduct any Tolls, parking fees,
The amount you have left is what you pay Self Employment Tax on. SE tax is how independent people earn towards retirement Social Security.
In California you will pay tax on your Notary income as they commission you and are due their share. You will only pay the % you pay for CA taxes.
Try to find a tax professional who is also a Notary to get all the right info.