You will need to include a resistor in series with the led(s).
LED's use a specific voltage (dependant on the colour, type)
So what you need to do is use the formula below to work out what resistor you need.
Series resistor = supply voltage - led voltage / led current
In your case supply voltage = 6v
led voltage = You can get this information from the supplier of the led's though generally it's Blue led = 3.5v, white led = 3.5v, red led = 2.3v, orange led = 3.8v, green led = 3.4v (these are for hi brightness led's)
led current = usually 20mA for most hi-brightness led's (this = 0.02A)
So for a blue led on 6v, you'd do this:
Rs=6v-3.5v/0.02=125
hm: closest higher value is 150
hm: so you'd need a 150
hm: resistor
but you need to check what the voltage drop of your particular led's is. If you want more than one, it's best to wire a separate resistor for each led. see the diagram attatched