Depends upon your accounting system. If you use a "first-in, first-out" approach, then the first block of stock you purchased is the first block of stock that you sold. With a "last-in, last-out" approach, then the most recent block purchased is the first block sold.
Regardless of the approach you select, enter each individual block sold as a separate transaction on Schedule D.
For example, if you purchased 100 shares of XYZ Corp on three separate occasions. You paid $10 per share for the first 100 shares, $12 a share for the next 100 shares, and then $14 a share for the last 100 shares you purchased. Let's say you sold all 300 shares in a single transaction at $20 per share. This sale is treated as three separate sales of 100 shares each. On the first sale, you realized a $1000 capital gain. On the second sale, you realized an $800 capital gain, and, on the third sale, you realized a $500 capital gain. The acquisition dates and the sale date determine whether your capital gain for each sale is long term or short term.
