Since I have no experience in electronics whatsoever, this was the toughest part for me to find out.
Other instructables had no good explaining on this subject.
So for everyone who's new to this electronics stuff and have no idea what they're doing, here is some explaining for the circuit.
1. How many LED's to use?
The average LED needs 3v to work. If you are going to put the LED's in series (like I did) you need to calculate how many LED's you can use with your adapter. The formula you can use is adapter output voltage / LED voltage = Total LED's
So if you use a 12v adapter with the 3v LED's its: 12 / 3 = 4 LED's
You can also put LED's parallel in the circuit. But I'm not getting into this, just so that this instructable stays simple and easy to understand for everyone. Let's just focus now on the LED's in series.
If you want to experience with more LED's, you can always calculate your LED's and resistors here.
2. The real voltage on a adapter.
Before I went building this circuit, I thought it would be useful to measure the real voltage on the adapter. The sticker on the adapter (photo 1) says the output 12v. But once I hooked it up on my multimeter, it shows that the actual output is around the 18v (photo 2).
So that means I can calculate the LED's again: 18 / 3 = 6 LED's.
Since I'm going to make the LED's in series I can use 6 LED's in my circuit.
3. The 3.5 audio jack plug
Which wire is what? That's what I was asking and trying to find out.
As you can see on photo 3, the plug itself has 3 metal parts, and 3 wires in the black protective layer.
On the photo I explained which wire is what.
Now with this information you can go to the next step, building the circuit.