Feeling seek today so will not talk on intro...sorry...
First select the leds for which you want to make the jig.
i took 5 mm leds, you can took leds of your choice.
Step 1: Measuring..
Bend the cathode or short leg of the led like the image above.
and take the measurement between two leds. (leg to leg)
leave enough leg length of led so you dont have to struggle to solder them.
Step 2: Selecting Board
Now you have to select the board for jig.
you can take wood piece of plastic piece like me.
Note : the board should have enough height so led doesn't touch ground, otherwise it will pop out when you give pressure on it. )
Now. draw lines like the image above. mine is 2 * 2. you should take leds measurement.
Step 3: Making Holes
Now make the desired holes where the horizontal & vertical lines are crossing each other.
i am making 4 * 4 * 4 led cube so i made 4 * 4 holes.
the diameter of hole should be same as diameter of leds.
in my case its 5mm.
Step 4: Check the Holes..
Put the leds in holes so you can check it if it fits in it or note.
if not, then refine it.
if yes, then now you can start to make your led cube. ^_^