Si no uso acondicionador, luego me cuesta peinarme.

Because this sentence is talking about a general, habitual situation:

• Si no uso acondicionador, luego me cuesta peinarme.
• If I don’t use conditioner, then it’s hard for me to comb/style my hair.

In Spanish, for this kind of real, repeated condition, you normally use the present tense in the si clause:

• Si estudio, aprendo más.
• Si como mucho, luego me siento mal.

So si no uso is the normal choice here.

Other forms mean different things:

• si no usara = if I didn’t use / if I were not to use
  This sounds more hypothetical.
• si no usaría is not correct after si in standard Spanish. Spanish normally does not use the conditional right after si.

Spanish often drops subject pronouns because the verb ending already shows who the subject is.

• uso = I use
• usas = you use
• usa = he/she/it uses

So Si no uso acondicionador already clearly means If I don’t use conditioner.

You could say Si yo no uso acondicionador if you want extra emphasis or contrast, but it is not necessary.

Yes. Acondicionador is a normal word for hair conditioner.

In context, Spanish speakers understand that it means conditioner for the hair, even though the sentence does not say para el pelo.

You may also hear:

• acondicionador para el pelo
• acondicionador del pelo
• sometimes simply suavizante in some contexts, though acondicionador is the safest general word

So for a learner, acondicionador is a very good word to use.

Because after usar, Spanish often does not use the article when speaking in a general sense about a substance or product.

So:

• uso acondicionador
• uso champú
• uso crema solar

This sounds natural when you mean I use conditioner/shampoo/sunscreen in general.

If you say uso el acondicionador, it sounds more like you mean a specific conditioner, for example:

• Uso el acondicionador que me recomendaste.
  I use the conditioner you recommended to me.

Here luego means then, afterwards, or later.

So:

• Si no uso acondicionador, luego me cuesta peinarme.
• If I don’t use conditioner, then afterwards it’s hard for me to comb/style my hair.

In Spain, luego is very commonly used this way.

A useful note: in some contexts, luego can also mean later or even see you later, as in:

• Hasta luego.

And in some parts of the Spanish-speaking world, luego can sometimes mean soon, so context matters.

Yes, you could.

These are both possible:

• Si no uso acondicionador, luego me cuesta peinarme.
• Si no uso acondicionador, después me cuesta peinarme.

Both mean something like then/afterwards it’s hard for me to comb my hair.

That said, luego feels very natural in this sentence.
Después is also fine, but it can feel slightly more explicitly time-based: after that.

Yes, literally costar means to cost, but very often it also means to be difficult for someone.

So:

• Me cuesta peinarme = It’s hard for me to comb/style my hair
• literally: It costs me to comb/style my hair

This is a very common Spanish structure:

• Me cuesta levantarme temprano.
  It’s hard for me to get up early.
• Le cuesta hablar en público.
  It’s hard for him/her to speak in public.

So me cuesta + infinitive is a useful pattern to learn.

Because they are doing two different jobs.

1. me cuesta

This me means for me / to me in the expression costarle a alguien + infinitive:

• Me cuesta = it’s hard for me
• Te cuesta = it’s hard for you
• Nos cuesta = it’s hard for us

2. peinarme

This me is part of the reflexive verb peinarse, meaning to comb/brush/style one’s hair.

• peinarme = to comb my hair / to do my hair
• peinarte = to comb your hair
• peinarse = to comb his/her/their hair

So in me cuesta peinarme, the first me belongs to costar, and the second me belongs to peinarse.

Because peinarse usually means to comb or style one’s own hair, while peinar by itself usually needs an object.

Compare:

• Voy a peinarme.
  I’m going to comb/style my hair.
• Voy a peinar a mi hija.
  I’m going to comb my daughter’s hair / do my daughter’s hair.

In your sentence, the speaker is talking about their own hair, so peinarme is the natural form.

Because Spanish often uses a reflexive verb where English uses my hair / your hair / his hair.

So instead of saying:

• me cuesta peinar mi pelo

Spanish more naturally says:

• me cuesta peinarme

The idea of my own hair is already understood from the reflexive pronoun me.

You can say el pelo if you want to be more explicit:

• me cuesta peinarme el pelo

That is also correct. But me cuesta peinarme sounds very natural and efficient.

It can cover a broader idea than English comb.

Depending on context, peinarse can mean:

• to comb your hair
• to brush your hair
• to do/style your hair

So in a sentence about conditioner, peinarme is best understood as something like to manage/style/comb my hair after washing it.

That is why the sentence sounds natural even if English might prefer brush my hair or do my hair in some situations.

Because in Spanish, no normally goes directly before the conjugated verb.

• No uso acondicionador.
  I don’t use conditioner.
• No tengo tiempo.
  I don’t have time.
• No quiero salir.
  I don’t want to go out.

So in the conditional clause:

• Si no uso acondicionador...

the no comes right before uso.

It is normal and helpful here.

The sentence starts with a si-clause:

• Si no uso acondicionador, ...

and then gives the main clause:

• ... luego me cuesta peinarme.

A comma is commonly used after an introductory subordinate clause like this, especially in writing, because it makes the structure clearer.

You may sometimes see similar sentences without a comma in informal writing, but with a comma is a good standard choice.

Yes. That is a very good alternative.

Compare:

• me cuesta peinarme = it’s hard for me to comb/style my hair
• me resulta difícil peinarme = I find it difficult to comb/style my hair

Both are correct.
Me cuesta peinarme is very natural and common in everyday speech.
Me resulta difícil peinarme sounds a bit more formal or explicit.

Yes, it sounds natural in Spain.

• Si no uso acondicionador, luego me cuesta peinarme.

This is a normal, everyday sentence in Peninsular Spanish.
Its vocabulary and grammar are standard and natural.

A Spaniard might also say slightly different versions, such as:

• Si no me pongo acondicionador, luego me cuesta peinarme.
• Si no uso suavizante, luego me cuesta peinarme el pelo.

But your original sentence is perfectly good Spanish for Spain.