Paying estimated tax.

https://www.irs.gov/businesses/small-businesses-self-employed/estimated-taxes

Where, see link for online IRS payment website. 

When, four quarterly deadline in a year.  See link above. 

How much, assuming your 20K investment turned into 140K, and you cashed out 100K, cashed 71% of total value, so the cost basis of your 100K is 71% of original 20K = 14K.  Thus the taxable capital gain is 100K - 14K = 86K.  

Estimated tax = 86K * 15% = $12900 ~ $13K.