How do I handle differences between the blueprint and the real site?

You can read most plans now and picture what they mean. It is when the drawing and the actual site disagree that you get tripped up, and that is one of the hardest parts of the trade, because it asks you to trust your own read over the paper in front of you. Here is the thing worth knowing: plans are intentions, and the real world is stubborn. Walls end up where they end up, old work was done wrong, measurements drift. The experienced hand does not expect the drawing to match reality. They expect the gap and go looking for it, treating the plan as a guide, not a gospel. When something does not line up, stop and verify on site before you build to the drawing. Measure it yourself. A discrepancy caught early is a small fix, one caught late is a costly one. You are not wrong to be tripped up now. You are learning the judgment that only real jobs can teach.