1-First make a simple outline like the one below
2- Now im not an art teacher, so im gonna explain this in simple graff terms.
if your doing 3D on the bottom of the piece, then draw straight and equal lines going down from the outside of each and every bottom corner of the piece that faces down (do not make lines withing the letters but on the outside of each letter, and make sure that each line is about the same length).
3- Now you want to connect each downward line. Do this but keep in mind the shape of the letter as you do it (meaning if you look at the bottom of the letter "B" as i connect the down lines i still draw any straight or curved line just like the shape of the actual letter).
4- Now that you did the 3D, you now want to add some type of style in it.
Below i did a style called GRIDS.
on each side of a 3D box. Ink in a phat downward line, then a meium one, then a thin one. Repeat inside every 3D box on both sides of each box (unless there is no space for it, you can also use only two grid lines if thats all that fits.
5- Now here i get a lil more creative and connect, and combine grid lines to create a more stylish 3D. i won't bother explaining this, so use your imagination.
HOW TO DO BLOCK 3D 
Linear Mode
