How to detect straight lines in an image and clean them up?

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If lines have same (width) Use Erodefor some number of iteration until lines are removed then use Dilate for the same number of iteration until your text is the same as before.

• Erode result
• Dilate result

Since you said

I have completely 100% zero experience yet

this tutorial was meant to be a good intro. It even includes a step-by-step explanation of the code which will helps you understand the morphological operations you need to apply to segregate the lines. You'd need to run multiple tests to find out which values makes sense for your case; lines at a random angle. The tutorial covers line detection at both horizontal and vertical axis.

With that understanding, you can start exploring houghLines, houghLinesP and even lineSegmentDetector to solve the problem as a whole.