IRS guidelines for mileage logs allow for some level of flexibility based on your circumstances. It is always a good idea to have a beginning odometer reading from the beginning of the year, an ending odometer reading from the end of the year and a full detailed log of every time you used your auto for business including the business purpose and all details of the travel.
However, this is not always available. For instance, if you are starting to track this detail sometime in the middle of the year and do not know what your beginning odometer reading was, but you did use your car for business from the beginning of the year until now, there are ways to make reasonable estimates of the miles you drove for business.
To come up with a beginning odometer reading, look for auto repair receipts or oil change receipts from a time as close to the beginning of the year as possible. These receipts usually have good odometer readings on them. If, for example, you have one from November and one from February, you can estimate your beginning balance as of January 1 by averaging the two or by using some other reasonable method of arriving at a beginning balance. Whatever method you choose, document how you arrived at the beginning odometer reading so you can provide that to your tax preparer at tax time.
As for tracking mileage throughout the year, the very best method is to have a log for each time you used your auto for business. But if you are starting to track your mileage sometime during the year and do not have records for the first part of the year, the next best method is to track your business mileage for the remainder of the year and then working with your tax preparer, reasonably estimate what the mileage was for the missing months based on the actual logs for the period you were able to track. You might consider discussing this with your tax preparer now so you can plan how he or she is comfortable helping you get a reasonable estimate for the period of time for which you do not have detailed logs.
Then, by all means, make sure you have a good ending odometer reading. This will become the beginning odometer reading for next year. As you enter mileage logs, you can enter the ending odometer on each transaction if you would like, but this is not necessary. The only purpose for that is to update the estimates reports and is not used for any other tracking purposes as it will get updated with each new "ending" entry you enter. The final log at the end of the year must have a good ending odometer reading for all the reports to be accurate.
If you decide on a reasonable estimate for the business mileage during the period of time you were not tracking it, you can enter this year-to-date business mileage amount in one log so the estimates used in reporting for the remainder of the year are more accurate. To do this, simply enter the end of the estimate period as the date of the log, enter "Beginning Balance estimate" as the description and enter the total estimated miles in the "miles driven" field.