Get a poster paper.  Colour does not matter.

Draw a block letter on it.  Use a ruler and dont make it too small.  Mine is 15 cm wide by 21 cm tall.

Cut it  out.

Trace it on the poster paper.

Cut out the second one.  Be aware if your letter has to face a certain way to be seen correctly,  ex.  the letter B can only be correct like it is on the screen, not backwars like an A.

Measure long rectangles on your paper for the sides of your letter.

Cut them out.

Take one of the long rectangles and measure it against one side of your letter.  I numbered these so assembly was easier later.

Cut it out.

Do the same for the other sides of the letter.

Even the small sides need a piece cut out for them.

Here are mine all cut out.  Notice how they are all numbered.  This is the inside of the letter.

Flip over and decorate each piece.

Make it look cool.

Flip all the pieces over again and line up the numbers.

Use clear tape to connect the larger pieces together

Do the same for the smaller pieces.

Here I taped these ones so that they are standing up and forming the 3D shape of my letter.

Here I taped the bottom of my letter A.

I flipped both sides together and gently tapped them with clear tape.

Here it is all complete.