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.