Usage patterns
〈物〉を束にするto gather (things) into a bundle
I bundled up the old magazines and tied them with string.
〈物〉の束a bundle of (something)
There was a bundle of letters sitting on the table.
束で〈動詞〉to buy/sell/handle (something) by the bundle
This flower shop sells flowers by the bundle.
Expressions
束になるform a group; form a bunch; bunch up