Close-ups

Hello;
I just fell into the medium format world.
I have a PENTAX 645, 45MMF/2.8, 75MMF/2.8, 75MMF/2.8 ls, 150MMF3.5 AND HELICOID EXTENTION TUBE.
What I would like to know is how to make close-ups?

The extension tube goes between the camera body and the lens - the more you extend the tube the closer you will be able to focus. You can focus and compose your picture through the viewfinder in the normal way. Exposure can also be relied upon.