Katso näyttämöä tarkasti ja kerro, miltä se näyttää.

They are both second-person singular imperatives, used to give an instruction or command.

• katsoa → katso = look
• kertoa → kerro = tell

So the sentence is addressing one person directly: Look ... and tell ...

Finnish usually leaves out the subject pronoun in imperatives, so there is no sinä here. Adding sinä would sound unusually emphatic.

Näyttämöä is the partitive singular of näyttämö.

With katsoa meaning look at, Finnish often uses the partitive object, especially when the action is seen as ongoing or not producing a completed result. You are directing your attention toward the stage, not finishing or consuming it in any way.

So:

• katso näyttämöä = look at the stage
• not normally katso näyttämö

A useful comparison is:

• katsoa elokuvaa = to watch a movie
• katsoa elokuvan = to watch the whole movie through

In this sentence, näyttämöä is the natural choice.

Tarkasti is an adverb, meaning carefully, closely, or accurately.

• tarkka = an adjective: careful, precise
• tarkasti = an adverb: carefully, precisely

So:

• tarkka ihminen = a careful/precise person
• katso tarkasti = look carefully

The ending -sti often turns an adjective into an adverb, somewhat like -ly in English.

Because miltä se näyttää is a subordinate clause—more specifically, an embedded question after kerro.

Finnish normally puts a comma before subordinate clauses:

• Kerro, miltä se näyttää.
• I know, where he is would be wrong in English, but Finnish uses the comma much more regularly in this kind of structure.

So the comma is standard Finnish punctuation here.

Miltä is a case form of mikä.

In this sentence, it appears in the very common pattern:

• miltä näyttää = what ... looks like / how ... looks

This is not something you should translate too literally word for word. It is best learned as a structure.

Some similar patterns:

• Miltä se kuulostaa? = What does it sound like?
• Miltä se maistuu? = What does it taste like?
• Miltä hän näyttää? = What does he/she look like?

So here miltä se näyttää means what it looks like.

They do different jobs.

• se = the subject = it
• miltä = asks about the appearance/quality = what ... like

So:

• miltä se näyttää
  • miltä = what ... like
  • se = it
  • näyttää = looks

English compresses this into what it looks like, but Finnish keeps those roles separate.

Here it means look or seem, not show.

That is because the meaning of näyttää depends on the structure around it:

• näyttää jotakin = to show something
  • Hän näyttää kuvan. = He/She shows the picture.
• näyttää joltain / miltä = to look like / seem
  • Se näyttää hyvältä. = It looks good.
  • Miltä se näyttää? = What does it look like?

So in miltä se näyttää, the verb clearly has the look/seem meaning.

Because in Finnish, a question word usually comes first in both direct questions and embedded questions.

So the normal order is:

• miltä
  • se
    • näyttää

This is the standard way to form the clause.

Compare:

• Miltä se näyttää? = What does it look like?
• Kerro, miltä se näyttää. = Tell (me) what it looks like.

A different order like se näyttää miltä would not work here.

Finnish often leaves out an indirect object like to me if it is obvious from the situation.

So:

• kerro, miltä se näyttää literally is closer to tell what it looks like
• but in natural English it is often understood as tell me what it looks like

If you wanted to make me explicit, you could say:

• kerro minulle, miltä se näyttää

But leaving out minulle is completely normal.

Not quite; they are close, but not the same.

• miltä se näyttää = what it looks like
  Focus: appearance, visual impression
• millainen se on = what it is like / what kind of thing it is
  Focus: general nature or characteristics

For a stage that someone is supposed to examine visually, miltä se näyttää is the more natural choice.

For example:

• Kerro, miltä näyttämö näyttää. = Tell me what the stage looks like.
• Kerro, millainen näyttämö on. = Tell me what the stage is like.

The second one sounds broader and less specifically visual.