Prepara la capa de drenaje a fin de que salga agua.

Yes. Prepara is the affirmative tú command of preparar.

• tú preparas = you prepare
• prepara = prepare! / get ready!

So the sentence is speaking directly to one person in an informal singular way.

If the speaker wanted to be formal with usted, it would be:

• Prepare la capa de drenaje...

If the speaker wanted to address you all in most of Latin America, it would be:

• Preparen la capa de drenaje...

Spanish often leaves out subject pronouns when they are clear from the verb form.

So instead of saying:

• Tú prepara...

Spanish simply says:

• Prepara...

In commands, this is especially normal. The verb form already tells you who is being addressed.

Literally, capa means layer, and de drenaje means for drainage or of drainage.

So:

• capa = layer
• drenaje = drainage

Together, capa de drenaje means drainage layer.

This is a very common Spanish pattern:

• capa de pintura = layer of paint
• sistema de riego = irrigation system
• tubo de escape = exhaust pipe

The structure noun + de + noun is extremely common in Spanish.

A fin de que means so that, in order that, or for the purpose that.

It introduces a purpose:

• Prepara la capa de drenaje a fin de que salga agua. = Prepare the drainage layer so that water comes out / can drain out.

It is correct, but it sounds more formal or technical than everyday speech.

In more natural everyday Spanish, many speakers would more often say:

• ...para que salga agua.

So:

• a fin de que = more formal, written, technical
• para que = more common in daily speech

Because a fin de que is followed by the subjunctive, not the indicative.

Here, the clause expresses purpose, not a simple statement of fact. In Spanish, purpose clauses with expressions like para que and a fin de que normally require the subjunctive.

So:

• salga = present subjunctive of salir
• sale = present indicative of salir

Compare:

• Lo hago para que funcione. = I do it so that it works.
• Sé que funciona. = I know that it works.

In your sentence, the speaker is talking about the intended result, so Spanish uses the subjunctive:

• a fin de que salga agua

Because the subject of the second action is different, or at least not expressed as the same person doing the first action.

A helpful rule:

• Use para + infinitive when the subject is the same.
• Use para que / a fin de que + subjunctive when there is a new subject or when the result is expressed in a separate clause.

Compare:

• Estudio para aprender.
  Same subject: I study / I learn

• Abro la ventana para que entre aire.
  Different subject in grammar: I open / air enters

In your sentence:

• Prepara... = you prepare
• salga agua = water comes out

Since the second part has its own subject, Spanish uses a fin de que + subjunctive.

Salga is the present subjunctive form of the verb salir.

The present subjunctive forms of salir are:

• yo salga
• tú salgas
• él/ella/usted salga
• nosotros/nosotras salgamos
• ustedes/ellos/ellas salgan

So in salga agua, the understood structure is:

• que salga agua = that water come out

The verb is singular because agua is treated as a singular mass noun here.

Spanish often places the subject after the verb, especially when introducing something that appears, happens, enters, comes out, etc.

So both of these are possible in Spanish:

• que salga agua
• que el agua salga

But que salga agua sounds more natural here.

This word order is very common with verbs like:

• entrar = to enter
• salir = to come out
• llegar = to arrive
• haber = there is/are

So the post-verb subject in this sentence is normal and idiomatic.

Here, agua is being used in a general, non-specific sense, like water in English.

So Spanish does not need an article:

• salga agua = water comes out / some water comes out

If you said el agua, it would sound more specific:

• que salga el agua = so that the water comes out

That might refer to a particular body of water already mentioned.

Also, remember that agua is a feminine noun, even though it often uses el in the singular:

• el agua fría
• las aguas frías

It takes el in the singular for pronunciation reasons, but it is still feminine.

Literally, yes, it is so that water comes out.

But depending on context, a more natural English translation might be:

• so that water can drain out
• so that water flows out
• so that water can escape

That is because salir often means to go out / come out / exit, but the best English wording depends on the technical context.

So the Spanish is straightforward, but the most natural English translation may vary.

Yes. In most situations, that would be the most natural substitution:

• Prepara la capa de drenaje para que salga agua.

This keeps the same basic meaning but sounds less formal and more conversational.

Use a fin de que when you want a more formal, precise, or technical tone. Use para que for ordinary speech in most cases.