Step 1 - Unscrew the locknut. Set aside the locknut and the seals underneath the locknut in a safe place.
Step 2 - Roll a chunk of plumbers putty into a ball and warm it up a little in your hands. Roll the putty out into a long string, like a snake about the width of a pencil.
Step 3 - Place the putty string around the underside of your basket strainer, along the flange, and push it down slightly so it doesn't fall off when you tip the strainer over.
Step 4 - Place the basket strainer into the sink drain opening and press down firmly. Don't press too hard, you want to avoid squeezing too much putty out from the around the flange.
Step 5 - Get underneath the sink (take the seals and locknut you set aside earlier with you). Slide the seals and locknut back over the basket strainer and tighten.