You go to a bank and open a tax account.  Deposit money every time you are paid.  Do not use this money for any other reason.  Keep careful and accurate records of how much cash you are paid every payday. 

Go to the IRS.GOV website and look at what records and accounting method they need so you can file your taxes at the end of the tax year.  You should be depositing at least 25% of the take-home pay in this tax account--if you don't use it all you'll have had a nice little savings to use after your taxes are finally paid. 

you make sure you keep track of how much and you will report it on sch C with your 1040
if the 'net' amount(income less business deductions) is $400 or more you will calculate your self employment tax on sch SE
all these are found at www.irs.gov as well as a very good publication to help you understand the tax system, a # 17
you can read it, down load it or order one a 1 800 829 3676