how to create squares in JOSM.
I’m assuming you are thinking of the geometric meaning (a figure with right angles and four sides of equal length) of the word. In that case, you can do the following:
Draw a way, and watch the length in the status bar at the bottom of the window before you place the second node.
Switch to the “create areas (x)” tool and extend the . Again, watch the length in the status bar to make sure it is the same as before.
Look in the Tools menu of JOSM … there is a tool “make rectangular” or similar … press “Q” to use it with an object with 4 corners or more that you have roughly drawn freehand in JOSM.