Depois do almoço, eu vou descansar no sofá.

Because de + o contracts to do in Portuguese.

• de + o = do (masculine singular)
• de + a = da (feminine singular)
• de + os = dos, de + as = das
  Here almoço is masculine (o almoço), so it becomes Depois do almoço.

They’re very similar: both mean after lunch.

• Depois do almoço is more common in everyday speech.
• Após o almoço can sound a bit more formal or “written,” though it’s still normal.

Because Depois do almoço is a fronted time expression (an introductory adverbial phrase). In Portuguese, it’s very common to use a comma after an introductory phrase like that, especially in writing:

• Depois do almoço, eu vou descansar...
  In casual texting or speech, people may omit the comma, but it’s standard to include it.

You can drop it most of the time. Portuguese verb endings usually show the subject, so both are natural:

• Depois do almoço, vou descansar no sofá. (very common)
• Depois do almoço, eu vou descansar no sofá. (adds emphasis/contrast, or clarity)

In modern Brazilian Portuguese, descansar is normally used without me:

• vou descansar (standard)
  Vou me descansar exists but tends to sound regional, old-fashioned, or nonstandard to many Brazilians. You will hear me more with some other verbs (e.g., vou me deitar = I’m going to lie down).

no is a contraction of em + o:

• em + o = no (in/on the)
  So no sofá means on the couch/sofa (or sometimes in the sofa in a literal sense, but here it means resting on it).

Two things: vocabulary and gender.
1) The sentence chooses sofa as the place you’ll rest. If you mean bed, you’d say na cama.
2) cama is feminine (a cama), so it contracts as em + a = na:

• vou descansar na cama = I’m going to rest in/on the bed.

By itself, it usually means after lunch (at that time), and context decides whether it’s a one-time plan or a routine.

• In this sentence, vou descansar suggests a specific plan: After lunch, I’m going to rest...
  If you wanted to emphasize “after I have lunch,” you could say something like:
• Depois que eu almoçar, vou descansar no sofá. (After I have lunch, I’ll rest on the couch.)

Yes, Portuguese is flexible, though some orders sound more natural. Common options:

• Depois do almoço, vou descansar no sofá. (most natural)
• Depois do almoço, vou descansar no sofá da sala. (adds detail)
  Putting no sofá earlier is possible but can sound marked/emphatic:
• Depois do almoço, no sofá eu vou descansar. (emphasis on “on the couch”)

A rough Brazilian Portuguese guide (stress in CAPS):

• de-POIS do al-MO-sso, eu VOU des-can-SAR no so-FÁ
  Notes:
• depois often sounds like de-POIS.
• almoço has a “soft” ç sound like s.
• Final r in descansar is often softly pronounced (or nearly dropped) in many Brazilian accents.