Every schematic symbol needs a footprint: the exact pattern of copper pads, holes, and silkscreen that the physical component solders onto. The symbol says what the part does; the footprint says how it attaches. A wrong footprint (wrong pad spacing, wrong pin order, wrong package) produces a board that looks perfect and does not work.
Match the footprint to the part you will actually buy
Pick real, in-stock parts early, ideally with a distributor part number, and use the footprint that matches that exact package. A resistor is not just a resistor: an 0402 and an 0805 need different land patterns. Pull the recommended land pattern from the manufacturer datasheet whenever one is given, rather than trusting a generic library part.
Verify before you commit
Before routing, check each footprint against its datasheet: pad dimensions, pin one location, courtyard, and 3D height if space is tight. For anything unusual, print the footprint at 1:1 scale and lay the physical part on top. This one habit prevents the most expensive class of beginner mistake, the kind you only discover when parts will not fit the board you already paid to have made.