Sling is used after these nouns:
shoulder

loosely
carelessly, casually
His jacket was carelessly slung over one shoulder.

across, around, in, on, over, round (esp. BrE), etc.

sling mud at sb (= criticize or accuse sb to damage their reputation)
sling sth around sb's shoulders
He slung his arm around my shoulders.

sling sth over your shoulder
She slung her bag over her shoulder.

Sling is used with these nouns as the object:
backpack, bag, jacket, pack, rucksack