Hi
It's really, really very easy.
Find the idea of circuit here.
**broken link removed**
Now, through your program, implement the following.
1. Keep unit's place display active, and others disabled. Send your data (Unit's place) to it.
2. Wait 2ms.
3. switch it off, and make ten's place data available at port.
4. Make ten's place diaplay active, and others off.
5 Do the same for next displays.
6. Repeat in cycles.
Better you'd make a separate function for this operation, and call it continuous infinite loop, while rest of your program should work through interrupts.
Hope it helps
Good Days