Please forgive me if I have overlooked it somehow. But I would like to make a certain top untuckable. I believe I could do this via SimPE. I would appreciate if someone could direct me to a tutorial for this topic.

Thank you

You don't need SimPE, just BodyShop and a paint program. What makes a top "untuckable" is how long the shirt is. So just change the texture and alpha of the shirt so it is long, past the waist, down to the sims hips. Use the alpha from existing untuckable shirts (except for the sleeves, you might have longer/shorter sleeves). The alpha is the black and white image in your BodyShop project folder. Where the alpha is black, your texture will be completely see through. Where it is white, your texture will be opaque.