Hoy quiero hacerme una trenza antes de salir.

Because me shows who the action is for: for myself.

In hacerme una trenza, the idea is literally something like to make myself a braid. In natural English, that usually becomes to braid my hair or to do my hair in a braid.

So:

• hacer una trenza = to make a braid
• hacerme una trenza = to make myself a braid / to braid my own hair

The me is very important here.

Here, me means to myself or for myself.

It is not quite the same as a fully reflexive idea like I wash myself, but it works similarly from a learner’s point of view. It tells you that the speaker is doing the hairstyle on their own hair.

Compare:

• Le hice una trenza a mi hija = I braided my daughter’s hair / I made my daughter a braid
• Me hice una trenza = I braided my own hair / I made myself a braid

Because Spanish often uses the pattern:

conjugated verb + infinitive

Here:

• quiero = I want
• hacerme = to do for myself / to make myself
• salir = to leave / to go out

So querer + infinitive works like to want to do something in English.

Examples:

• Quiero comer = I want to eat
• Quiero dormir = I want to sleep
• Quiero hacerme una trenza = I want to braid my hair

And after antes de, Spanish also uses an infinitive when the subject stays the same:

• antes de salir = before leaving / before going out

Yes, absolutely. Both are natural:

• Hoy quiero hacerme una trenza...
• Hoy me quiero hacer una trenza...

The meaning is basically the same.

The difference is mostly about placement and emphasis:

• quiero hacerme sounds a bit more neutral
• me quiero hacer brings the pronoun earlier, which can feel slightly more conversational or give a tiny bit more prominence to me

Spanish allows object/reflexive pronouns either:

• attached to the infinitive: hacerme
• or before the conjugated verb: me quiero hacer

Both are correct.

Because Spanish often uses the indefinite article in this kind of expression.

hacerse una trenza means to do one’s hair in a braid or to make oneself a braid. The possession is already understood because of me.

So mi is usually unnecessary.

• Me hice una trenza = I braided my hair / I made myself a braid

Using mi trenza would sound less natural in this context unless you mean a very specific braid already being talked about.

Because trenza already strongly suggests hair, so Spanish does not need to mention it.

Hacerse una trenza is a normal expression by itself. It naturally implies in your hair.

You could also say:

• Quiero trenzarme el pelo = I want to braid my hair

That version explicitly mentions the hair. But hacerme una trenza is perfectly natural and does not need pelo.

Both are natural, but they focus slightly differently:

• hacerme una trenza = to do myself a braid; focus on the resulting hairstyle
• trenzarme el pelo = to braid my hair; focus more directly on the action done to the hair

In everyday use, they often translate the same way in English.

Examples:

• Voy a hacerme una trenza = I’m going to do my hair in a braid
• Voy a trenzarme el pelo = I’m going to braid my hair

Because after antes in this structure, Spanish needs de before an infinitive.

So the pattern is:

antes de + infinitive

Examples:

• antes de comer = before eating
• antes de dormir = before sleeping
• antes de salir = before leaving / before going out

So antes salir is not correct here.

It can mean either, depending on context.

In this sentence, antes de salir could mean:

• before leaving
• before going out

If someone is getting ready at home, both are possible. Spanish salir covers both ideas more broadly than English sometimes does.

No. Spanish usually leaves out subject pronouns when the verb already makes the subject clear.

• quiero already tells us the subject is I
• so Hoy quiero hacerme una trenza... is complete and natural

You could say:

• Hoy yo quiero hacerme una trenza...

but that would usually add emphasis or contrast, for example:

• Hoy yo quiero hacerme una trenza, no un moño
  = Today I want to do a braid, not a bun

No. Spanish word order is flexible.

All of these are possible:

• Hoy quiero hacerme una trenza antes de salir.
• Quiero hacerme una trenza hoy antes de salir.
• Antes de salir, hoy quiero hacerme una trenza.

The first one is the most straightforward and natural in many situations.

Putting Hoy first is very common because it sets the time frame right away.

Because the speaker wants one braid.

If they wanted more, Spanish would change the number:

• hacerme una trenza = make myself one braid
• hacerme dos trenzas = make myself two braids
• hacerme unas trenzas = do some braids / braid my hair in braids

So the singular is just matching the hairstyle being described.

Yes, hacerse una trenza is perfectly natural in Spain.

Another very common option is:

• trenzarse el pelo

In Spain, trenza is the normal word for braid/plait. In British English, learners may think of plait; in American English, usually braid. The Spanish word is still trenza.

So for Spain, these are both good:

• Quiero hacerme una trenza
• Quiero trenzarme el pelo

Not in this sentence, because salir does not take me here.

The me belongs with hacer:

• hacerme una trenza = to make myself a braid

It does not belong with salir, because the meaning is not to leave myself or anything like that.

So:

• quiero hacerme una trenza antes de salir = correct
• attaching me to salir here would change the structure and would not express the intended meaning

Not exactly.

• peinarme = to comb my hair / to do my hair in a general sense
• hacerme una trenza = to make myself a braid specifically

So peinarme is more general, while hacerme una trenza is specific.

Compare:

• Quiero peinarme antes de salir = I want to do/comb my hair before leaving
• Quiero hacerme una trenza antes de salir = I want to braid my hair before leaving