Rippley

Ok - so I am moving into a new apartment here in Beautiful Copenhagen. The walls are all white, which is fine with me. However, In one hallway there's a fairly long wall where I want to put to horizontal stripes about a foot and half apart, and then fill the space with pictures. Now, how do I get those lines to be level?

Any and all suggestions are appreciated. I am NOT a DYI maven, by the way...

Thanks,
Ripp

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