Put on your safety goggles and cut the MDF board down to the size you wish for your bar top. This depends on how large you would like the bar to be.
Stack your masonry blocks as tall as you want--about waist high is sufficient.
Make another stack of blocks just as high and directly behind the first stack of blocks.
Make two more stacks of blocks and place them directly next to the first two stacks. You should now have two rows of stacked masonry blocks side by side. This will support one side of the bar top.
Make two more rows of stacked blocks of even height, right next to one another, to support the other end of the bar.
Place the cut piece of MDF board on top of the stacks and apply a layer of varnish to the wood to finish it.