I congratulate you on the increased earnings; however, remember that taxman is waiting around the corner. In case you expect to owe more than $1,000 to the IRS, you have to prepay it in advance. Otherwise, you will receive the underpayment penalty issued by the government, thus giving the money straight into the sewage pipe.
For this purpose, you will have to estimate your total gross income, deductions, and credits for a whole year. After that, you will have to subtract the tax withholding from any regular job you performed during the period from the total amount of taxes you will pay. If the amount received exceeds $1,000, you should take this amount and divide it by four to make payments in quarters.
The difficulty in this situation for freelancers is the tax on self-employment. As a contractor, you will not only have to pay income tax but also your share of taxes for Social Security and Medicare; thus, you will have to deduct 15.3% from your income.
Worksheet 1040-ES should be completed in such situations; however, some of the tax preparation software, including TurboTax and H&R Block, will make everything easier. Set reminders for April 15, June 15, September 15, and January 15, or otherwise, you will get into trouble with the authority. Having 25-30% saved from each paycheck will prevent you from an unpleasant surprise in April.