Mi siedo sul divano, mi slaccio gli stivali e bevo un tè caldo.

Because both verbs are being used in a reflexive/pronominal way.

• sedersi = to sit down
• slacciarsi = to unlace something on oneself / take off by unfastening

So:

• mi siedo = I sit down
• mi slaccio gli stivali = literally I unlace myself the boots, but in natural English: I unlace my boots

That second one can feel strange to English speakers. Italian often uses:

• a reflexive pronoun (mi, ti, si, ci, vi)
• plus a definite article (gli stivali = the boots)

where English would usually say my boots.

They are related, but not used in exactly the same way.

• sedere = to sit
• sedersi = to sit down

In this sentence, mi siedo suggests the action of taking a seat, not just already being seated.

Examples:

• Siedo qui. → uncommon in everyday speech
• Mi siedo qui. → I sit down here / I’m going to sit here

In practice, learners will very often meet sedersi when talking about sitting down.

With a conjugated verb, reflexive pronouns normally go before the verb:

• mi siedo
• ti siedi
• si siede

This is the standard pattern in Italian.

If the verb is in the infinitive, the pronoun can attach to the end:

• voglio sedermi = I want to sit down

So in your sentence, mi siedo is normal because siedo is a fully conjugated verb.

Sul is a contraction of:

• su = on
• il = the

So:

• su + il = sul

Therefore:

• sul divano = on the sofa

This is very common in Italian with prepositions and articles:

• a + il = al
• di + il = del
• in + il = nel
• su + il = sul

Because Italian usually drops the subject pronoun when it is already clear from the verb ending.

• mi siedo already tells you the subject is I
• bevo also clearly means I drink

So io is not necessary.

You could say:

• Io mi siedo sul divano...

but that usually adds emphasis, contrast, or clarity, for example:

• Io mi siedo sul divano, tu resta pure lì.
  I’m sitting on the sofa; you stay there if you want.

Because when the owner is obvious, Italian often uses:

• a reflexive pronoun
• plus the definite article

instead of a possessive adjective.

So:

• mi slaccio gli stivali = I unlace my boots

This pattern is very common with:

• clothing
• body parts
• personal items closely connected to the person

Examples:

• Mi lavo le mani. = I wash my hands
• Si mette il cappotto. = He/She puts on his/her coat
• Mi tolgo gli occhiali. = I take off my glasses

Using i miei stivali is possible, but it sounds more emphatic, as if you really want to stress that they are my boots.

Italian often uses an article where English may omit one.

• bevo un tè caldo = I drink a hot tea / I have a hot tea
• bevo tè = I drink tea in a more general sense, like a habit or substance

So un tè caldo presents it as one cup of hot tea, a specific drink in that moment.

Also, notice the accent:

• tè = tea

The accent helps distinguish it in writing and reflects the standard spelling.

Because in Italian, descriptive adjectives often come after the noun.

• un tè caldo = a hot tea
• una casa grande = a big house

Also, caldo must agree with tè in gender and number:

• tè is masculine singular
• so the adjective is caldo

If the noun were feminine plural, the adjective would change:

• bevande calde = hot drinks

Because the present tense in Italian can describe:

• what someone is doing now
• a habitual action
• a sequence in a narrative style

So this sentence can mean something like:

• I sit down on the sofa, unlace my boots, and drink a hot tea

It sounds natural in Italian for describing a scene or a routine. English can also do something similar in narration.

If you wanted a completed past sequence, Italian would normally use a past tense instead, for example:

• Mi sono seduto sul divano, mi sono slacciato gli stivali e ho bevuto un tè caldo.

Yes.

• slacciare means to unfasten / unlace / undo
• togliere means to remove / take off

So:

• mi slaccio gli stivali focuses on undoing the laces or fastenings
• mi tolgo gli stivali focuses on taking the boots off

In real life, these actions are often connected, but the verbs highlight different parts of the action.

Not perfectly. A literal translation would sound awkward:

• I sit myself on the sofa, unlace myself the boots, and drink a hot tea

Natural English would be:

• I sit down on the sofa, unlace my boots, and drink a hot tea
• or more naturally in many contexts: I sit down on the sofa, take off my boots, and drink some hot tea

This is a good example of how Italian and English often express the same idea with different grammar, especially with reflexive verbs and possession.