As noted about, either increase your withholding at your regular job...
or do what I do, which is keep a spreadsheet with quarterly running total freelance income, and send in a quarterly check to the IRS. I don't even send a form, just a check with my SS and the note "2018 taxes Quarter #1" or whatever. Usually I have to estimate how much I will be earning for the last couple weeks of each quarter b/c the check goes in before the quarter ends.
You can run detailed calcs of how much you will owe. Rule of thumb used to be 20-30% of gross (b/c you have to cover both sides of the payroll tax, not just one). However, I have found that 15% of gross has been more than enough for me, given how low our tax rate has been the past few years. And taxes have gone down again, starting this year, so you might want to run numbers for yourself.