You can use the withholding calculator through the link below. Just treat each source of income as a "job". Also, your human resources department through the government should be a source of help for this as well.
<a rel="nofollow" target="_blank" href="
https://apps.irs.gov/app/withholdingcalculator/">https://apps.irs.gov/app/withholdingcalculator/</a>
