sich anschnallen
to buckle up
To put on and fasten the seat belt in a vehicle, such as a car or airplane.
Vor der Fahrt müssen sich alle Passagiere anschnallen.
Before the trip, all passengers must buckle up.

to buckle up
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to buckle up
To put on and fasten the seat belt in a vehicle, such as a car or airplane.
Vor der Fahrt müssen sich alle Passagiere anschnallen.
Before the trip, all passengers must buckle up.

'Anschnallen' (to buckle up) is a separable verb. The prefix 'an-' comes at the end of the sentence in a main clause. In a subordinate clause or infinitive with 'zu', the verb is not separated.
Bitte schnallen Sie sich an.
Please buckle up.
The verb is used in different ways: 'sich anschnallen' means that you fasten your own seat belt. 'Jemanden oder etwas anschnallen' means that you secure another person or an object.
Ich schnalle mich vor jeder Fahrt an.
I buckle up before every trip.
When you fasten your own seat belt, you must use the appropriate reflexive pronoun (mich, dich, sich, uns, euch). A sentence like 'Ich schnalle an' is incomplete.
Schnallst du dich bitte an?
Will you please buckle up?
The request to buckle up is very often expressed in the imperative. The most common form is 'Schnall dich an!' (Buckle up!) or, in the polite form, 'Schnallen Sie sich bitte an!' (Please buckle up!).
Kinder, schnallt euch an, wir fahren los!
Kids, buckle up, we're leaving!
to solve / loosen
'lösen' and 'anschnallen' can be opposites when talking about belts or straps. 'anschnallen' means that you fasten a belt or strap and thereby secure yourself, a person, or an object. 'lösen' then means that you open or loosen that belt or strap again. A typical situation is in a car or airplane: before the trip, you buckle up; after the trip, you unfasten the belt.

Im Auto schnallt sie das Kind sorgfältig an.
In the car, she carefully straps the child in.
Nach der Landung löst Frau Wagner ihren Sicherheitsgurt.
After landing, Ms. Wagner unfastens her seat belt.