Antes de firmar, conviene fijarse en los detalles del contrato.

Antes de + infinitive is a very common Spanish structure meaning before doing something.

• antes de firmar = before signing
• antes de salir = before leaving
• antes de comer = before eating

Spanish uses de before an infinitive after antes.
So you say:

• antes de firmar
• not antes firmar

If there is a full clause with a conjugated verb, then Spanish often uses antes de que + subjunctive:

• Antes de que firmes, lee todo bien. = Before you sign, read everything carefully.

Because after antes de, Spanish normally uses the infinitive when the subject is general or the same as the main action.

• antes de firmar = before signing

Here, Spanish is talking about the action in a general way, not about a specific conjugated subject like you sign or he signs.

Quick comparison:

• antes de firmar = before signing
• antes de que firmes = before you sign
• firmado = signed / having signed
• firmando = signing

So firmar is the normal form here.

Conviene comes from convenir, and here it means something like:

• it is advisable
• it is a good idea
• it is wise
• it is in your interest

So conviene fijarse... means:

• it’s a good idea to pay attention...
• you should look carefully...
• it is advisable to notice/check...

It is a slightly impersonal, general way of giving advice. It sounds natural and fairly neutral.

Because conviene is a verb, while conveniente is an adjective.

• conviene = it is advisable / it suits / it is a good idea
• conveniente = convenient / advisable

So:

• Conviene fijarse en los detalles.
  = It’s advisable to pay attention to the details.

But you could also say:

• Es conveniente fijarse en los detalles.
  = It is advisable to pay attention to the details.

Both are correct.
Conviene is just more compact and very common.

This is an impersonal use. Spanish often leaves out a subject when English would use something like it or you in a general sense.

• Conviene fijarse... = It is advisable to pay attention...
• It can also feel like You should pay attention..., but in a general, less direct way.

There is no specific person stated. It is just general advice for anyone in that situation.

Because fijarse en is a set expression meaning:

• to notice
• to pay attention to
• to look carefully at

By contrast, fijar by itself usually means:

• to fix
• to set
• to fasten
• to establish

Examples:

• Fijó la fecha de la reunión.
  = He set the date of the meeting.

• Fíjate en eso.
  = Notice that / Look carefully at that.

So in this sentence, fijarse en los detalles is the correct expression.

In fijarse en, the se is part of the verb expression. You should really learn it as a whole unit:

• fijarse en algo

It does not translate word-for-word neatly into English. It helps form the meaning to notice / to pay attention to.

Conjugated examples:

• Me fijo en los detalles. = I pay attention to the details.
• Te fijas en todo. = You notice everything.
• Hay que fijarse en la letra pequeña. = You have to pay attention to the small print.

So here, fijarse is simply the infinitive form of that pronominal verb.

Because the standard expression is fijarse en.

• fijarse en algo/alguien = to notice something/someone, to pay attention to something/someone

Examples:

• Fíjate en el precio.
• Me fijé en su tono de voz.
• Debes fijarte en las condiciones.

This is just the preposition that goes with the verb, so it is best memorized together:

• not just fijarse
• but fijarse en

Del is the contraction of de + el.

• de el contrato → del contrato

So:

• los detalles del contrato = the details of the contract

This contraction is mandatory with de + el, except when el is part of a proper name.

Examples:

• el final del libro = the ending of the book
• la puerta del coche = the door of the car

But:

• de El Escorial stays separate because El Escorial is a name

Spanish often uses the definite article more than English does.

• los detalles del contrato literally = the details of the contract

In English, depending on context, you might say:

• the contract details
• the details of the contract
• details in the contract

Spanish naturally uses los detalles here because it refers to the specific details belonging to that contract.

You could say it, but it is not quite the same.

• mirar = to look at
• fijarse en = to pay attention to / to notice / to look carefully at

So fijarse en los detalles suggests more than just physically looking. It implies careful attention.

Compare:

• Mira el contrato. = Look at the contract.
• Fíjate en los detalles del contrato. = Pay attention to the details of the contract.

In this sentence, fijarse en is better because the idea is careful scrutiny before signing.

It is fairly neutral and works well in both formal and semi-formal contexts. It sounds a bit more polished and less direct than an imperative.

Compare:

• Conviene fijarse en los detalles del contrato.
  = It is advisable to pay attention to the contract details.
  Neutral, general advice.

• Fíjate en los detalles del contrato.
  = Pay attention to the contract details.
  Direct instruction to one person.

• Hay que fijarse en los detalles del contrato.
  = One has to / You have to pay attention to the contract details.
  Also common and general.

So conviene is a good choice when giving calm, general advice.

Because Antes de firmar is an introductory phrase placed at the beginning of the sentence.

Spanish often uses a comma after this kind of opening element, especially when it sets the time or context for the main statement:

• Antes de salir, apaga la luz.
• Después de comer, fuimos a dar un paseo.

The comma helps separate the introductory part from the main clause:

• Antes de firmar, | conviene fijarse en los detalles del contrato.

It makes the sentence clearer and more natural to read.

Yes. Here are a few natural alternatives, each with a slightly different tone:

• Antes de firmar, es conveniente fijarse en los detalles del contrato.
  More explicit, slightly more formal.

• Antes de firmar, hay que fijarse en los detalles del contrato.
  Stronger sense of general necessity.

• Antes de firmar, debes fijarte en los detalles del contrato.
  Direct advice to you.

• Antes de firmar, conviene revisar bien el contrato.
  Slightly broader: it’s advisable to review the contract carefully.

All are correct, but the original sentence is very natural and idiomatic.