Bring up the cheat prompt (ctrl+shft+c), type in "boolprop testingcheatsenabled true"(no quotes), and while your sim is not doing anything (turn of free will to make this easier) shift-click your sim, click 'more', then click 'plan outfit'. Buy some clothes for the sim (you have to buy clothes for this to work), then click the check button, then choose the outfit for them to wear.