Ordinary small 3mm & 5mm coloured LEDs (red, green, yellow,
orange) run at about 2V, but white and blue LEDs run at about 3.3V.

When using a supply higher than their nominal voltage, LEDs require a resistor connected in series with them to control the current.
(Without the resistor, the LED will burn out immediately.)

LED brightness is determined by the current, normally up to about 30 mA maximum for 5mm LEDs.
LEDs are polarity sensitive, so they must be the right way round. The long lead is positive.

To calculate the required resistor for a particular supply voltage:
LED Voltage + Resistor Voltage = Supply Voltage

For example: To run one white LED (3.3V) from a 12V supply,

Resistor voltage will be: 12V ─ 3.3V ═ 8.7V
Resistor value will be: 8.7V ÷ 30 mA ═ 290 Ohms
So we would use a 270 Ohm resistor (minimum value).
(As only certain values are made, close to calculated value is ok.)
A higher value resistor will reduce the brightness.

To adjust LEDs to your liking, use the minimum value resistor plus a 1k ohm potentiometer (variable resistor) in series, twiddle to the desired brightness.
Measure the total resistance, resistor plus potentiometer, with a multimeter, then find a resistor (or two in series) to approximate that value of resistance.

You can buy very cheap potentiometers on Ebay, so you can set one to about the needed resistance, leave it in the circuit and tweak it slightly as required.