What's the best and easiest way to calculate Estimated Taxes? Do I really need to pay them if I have SSDI and rental income? How do I always remember to pay them?

Estimated Taxes seem very difficult for me to calculate every year. I have to pay them because of a small rental property and rental income. For some years, I don't get rental income for the whole year if a tenant moves out, and a new tenant needs to be found. So I have to estimate income that can be inconsistent from year to year. What's the easiest and best way for me to calculate my Estimated Taxes as accurately as possible? 

 

I also receive SSDI payments. Do I really need to pay Estimated Taxes if I have SSDI and rental income (not always for the full year)? I also have small-ish amounts from interests/dividends. My filing status is single with no dependents.