The easiest way is to create two strings of three LEDs. Assuming the cheap Red, Yellow, or Green LEDs each string will need a 150 Ohm resistor. Keep in mind the LED's are polarized so they can only be connected one way.

Here's how the math works.

Each LED probably needs about two volts. So three in series (end to end) is 3LEDS X 2Volts= 6 Volts

You're using a 9 volt battery, so 9 Volts -6 volts= 3 volts. That's the voltage across the resistor.

The standard red, yellow, or green LEDs require 10 to 20 milliamps of current. More current is brighter (as long as you don't exceed the rating and burn them up). The current through the resistors and the LEDs will be the same since they are in series. So resistance=voltage/current. So the resistor has 3 Volts / 0.02 amps = 150 Ohms.

Duplicate that circuit for the other three LEDs. Connect the resistor from both strings to the positive of the battery and the negative leg of the last LED in both strings to the negative.

*VERY IMPORTANT: **You need to find the voltage and current rating for the LEDs you use and do the math accordingly. White LEDs, Blue LEDs, RGB LEDs all require different voltage and current. The math works the same. if it's a 3.2 Volt LED... 2 LEDs per string... that makes 6.4 volts... resistor Voltage is 9-6.4 V so 2.6 volts... say the current is still 20mA (0.02 Amps) that gives you a resistor of 130 ohms. It also means three strings of two connected the same way as the other example.*