Pickup an 8051 book and you shall find the answer. Or take a look at the example of AN529 at http://search1.microchip.com/10/appnot.../index.htm.

Two things to watch out:
1. Make sure your LED is common-anode or common-cathode by a multi-function meter.
2. The total current drain capability of x51 IO ports is always limited to dozens of mA. And a 7-segment LED could mean 7x10mA, which far exceeds the port's limit. that's why extra transistors are needed.

The rest (e.g., current limiting) is the same as driving any standalone LED.