How can I look like a pretty boy?

It’s hard to answer without knowing what you look like.

Please add more details. Permanently or just for Halloween?

What is your birth gender?

All of these things matter in making the transformation.

I would say a really good and professional haircut to start. Likely want to get some kind of product to style. Of course depends on what kind of “pretty boy” going for as that is a bit broad.

If you are, sporting a confident look, and stride, already helps.
Pretty is, obviously, very subjective.
Elvis was pretty, in my eyes, Brad Pitt less so.
For others they’re both handsome, for others still they both are meh.

The so called pretty boy look that I am talking about is associated with guys like Justin Bieber. I might link a pic but I don’t like posting them.

I don’t like using the word ‘pretty’ but it is a word associated with people like Bieber.