What's the best way to get feces off white cotton underwear?

a putty knife

soak them in a bleach mix

Never put face in underwear. Underwear for covering happy bits only

put in washer, add detergent..and wash them

Neutralize with a white vinegar solution (Mix one-third cup of white household vinegar with two-thirds cup of water). Apply white vinegar solution using a spray bottle and blot to remove excess moisture. Repeat step one with the neutral detergent to remove any stain that might still appear to be in the underwear.

Wash in hot water with good detergent. Pretreat any heavily soiled spots

Get a butter knife and smear it all over. That way when you're done washing them, the color matches all over.

burn them

You throw them away.