If you have multiple items that you want to purchase, the easiest thing to do
first is to build a "wanted list" (heart icon in the header bar). Everyone starts
with the "default wanted list", but you can make more if you want. Search for
a part via the main search and then once you find the piece that you need you
can click the "add to my wanted list" link. Do this for all the part you want.
After you have all of your wanted list(s) built, hover over the heart again and
click "buy". Then you can click all the lists you want and the filter will try
to locate buyers that have as many parts a possible.
Welcome to BL and don't hesitate to ask more question as you start your journey.