Your character can carry and equip one healing item in the
spot on your inventory. This can be either medical supplies or a healing potion. Press
to bring it out, and [cn_RT] to use it on yourself. For this achievement, however you need to use medical supplies on others (Potions don't help with this at all.) When near another character, hold [cn_LT] to use the medical supplies on them.
You will usually start with one medical supplies already on you, and can collect more during a mission, look for small boxes with a green outline.
I'm not sure if using supplies on Bot teammates count for this achievement, please let me know in a comment if they do.