Si no uso acondicionador, luego me cuesta más deshacer la trenza.

Because this is a real or possible condition: If I don't use conditioner...

With this kind of si clause, Spanish normally uses the indicative, not the subjunctive:

• Si no uso acondicionador, luego me cuesta más...
• If I don't use conditioner, then it's harder for me...

A useful pattern is:

• si + present indicative, present/future/command

Examples:

• Si tengo tiempo, voy.
• Si vienes mañana, te lo enseño.

If the sentence were more hypothetical or unlikely, Spanish would change both parts:

• Si no usara acondicionador, me costaría más deshacer la trenza.
• If I didn't use conditioner, it would be harder to undo the braid.

Spanish often uses the present simple where English might also use a more specific form.

Here, uso sounds natural because the sentence expresses a general habit or repeated situation:

• Si no uso acondicionador...
• If I don't use conditioner...

It is not talking about one exact moment when the speaker is in the middle of using it.
Estoy usando would mean I am using right now, and that would sound less natural in this general statement.

So this sentence is closer to:

• Whenever I don't use conditioner, it's harder afterwards to undo the braid.

Because acondicionador is being used in a general sense, like a product category.

So Spanish says:

• uso acondicionador = I use conditioner

This is similar to other general, non-count or category-style uses:

• Uso champú suave.
• No como carne.
• Bebo café.

You would use an article if you meant a specific conditioner:

• Si no uso el acondicionador que compré, luego...
• If I don't use the conditioner I bought, then...

So in your sentence, no article is perfectly normal.

Here luego means then, afterwards, or later.

So the idea is:

• Si no uso acondicionador, luego me cuesta más...
• If I don't use conditioner, then/afterwards it's harder for me...

It connects the first action with the later consequence.

In Spain, luego is very common for later/then.
In this sentence, it does not mean a distant future; it just means after that.

You could think of it as:

• first: I don't use conditioner
• later: undoing the braid is harder

Literally, it is something like:

• it costs me more to undo the braid

But in natural English, it means:

• it's harder for me to undo the braid
• I find it harder to undo the braid

This is a very common Spanish structure:

• costarle a alguien + infinitive

Examples:

• Me cuesta levantarme temprano. = It's hard for me to get up early.
• Le cuesta entender esto. = It's hard for him/her to understand this.

So in your sentence:

• me = for me / to me
• cuesta = is difficult / costs
• más = more
• deshacer la trenza = to undo the braid

The me shows who experiences the difficulty.

So:

• Me cuesta = It's hard for me
• Te cuesta = It's hard for you
• Le cuesta = It's hard for him/her

This is very common in Spanish with verbs like costar, gustar, molestar, and others, where the person affected is shown with an indirect object pronoun.

Compare:

• Me cuesta hablar en público.
• Te cuesta dormir.
• Le cuesta concentrarse.

So me does not mean myself here. It means to me / for me.

Deshacer la trenza is the natural way to say undo the braid.

• hacer una trenza = to make a braid
• deshacer una trenza = to undo a braid

Spanish often uses deshacer for reversing something that was made or done.

By contrast, desenredar means to untangle, especially when hair or strings are knotted or tangled:

• desenredar el pelo = to untangle the hair

So these are different ideas:

• deshacer la trenza = take the braid apart
• desenredar el pelo = remove tangles from the hair

Sometimes both things happen one after the other, but they are not the same action.

Spanish often uses the definite article where English prefers a possessive.

So if it is obvious whose braid we are talking about, Spanish can simply say:

• deshacer la trenza

Even though English would often say:

• undo my braid

This is very common with body parts, clothing, and things closely connected to the person.

Using mi trenza is possible, but it adds a bit more emphasis or explicitness:

• Si no uso acondicionador, luego me cuesta más deshacer mi trenza.

That is not wrong, but la trenza sounds more natural if the context already makes it clear that it is the speaker's own braid.

Usually yes. Trenza is the normal Spanish word for a braid or plait in hair.

Examples:

• Lleva una trenza. = She is wearing a braid.
• Me hice dos trenzas. = I made myself two braids.

In British English, plait is also common; in American English, braid is more common.
Spanish trenza covers that idea.

So in this sentence, deshacer la trenza means undo the braid.

Más means more, and here it implies more difficult.

So:

• me cuesta más deshacer la trenza

means:

• it is harder for me to undo the braid
• it costs me more effort to undo the braid

The comparison is understood from the context:

• harder than when I do use conditioner
• harder than usual
• harder than it would otherwise be

Spanish often leaves that comparison implicit if it is obvious.

Yes. Both are possible, but they focus slightly differently.

• usar acondicionador = to use conditioner
• ponerse / echarse acondicionador = to put/apply conditioner on oneself

So:

• Si no uso acondicionador... sounds like a general statement about the product you use.
• Si no me pongo acondicionador... focuses more directly on the act of applying it to your hair.

In everyday speech, especially when talking about hair products, both can sound natural.
The sentence you were given is completely idiomatic.

Yes, después would also be possible:

• Si no uso acondicionador, después me cuesta más deshacer la trenza.

Both luego and después can mean afterwards.
In this sentence, luego is slightly shorter and very natural.

A rough guide:

• luego = then / later / afterwards
• después = afterwards / later

In many everyday contexts like this one, they are interchangeable.