Take your favorite construction blocks of choice (LEGO™, PixelBlocks, Mega Bloks, K'NEX Bricks™ are all compatible) then start building. You can write messages, incorporate minifigs, build 3-D structures with gears, add motors... the mind boggles. The only limit is your imagination and the amount of weight you can deal with on the front of your shirt.
*The Brick Construction Shirt comes with the black baseplate on the front, but nothing to build with. You need to supply your own building blocks or bricks.