DIAGNOSIS You did not mess up. This is almost always a misaligned burner during assembly. The burner tube isn't seated correctly over the gas orifice.
FIX Turn gas off at the tank. Remove the cooking grates and flavorizer bars. Locate the problem burner. You'll see the burner tube slips over a brass spud (the orifice). Pull the burner tube straight back, then carefully re-seat it directly over the spud. It should click into place. Reassemble and test.
IF THAT DIDN'T WORK The spider web/insect nest issue can also cause a yellow, lazy flame on just one burner. Inspect and clean the burner ports on that specific tube.
HEADS UP A yellow flame means incomplete combustion, producing carbon monoxide. Do not use the grill until the flame is blue with yellow tips.