Learn Hardcore Spanish (Spain): Alzamos los cuadernillos antes del cosido. - We gather the signatures before the sewing.

Alzamos comes from alzar. Its basic meaning is to raise or to lift, but in technical contexts such as printing or bookbinding it can mean to gather, assemble, or put the sections in order. So here it may sound more specialized than a simple everyday lift.

Does alzamos mean we raise or we raised?

Grammatically, it can mean either one:

present indicative: we raise / we gather
preterite: we raised / we gathered

For many -ar verbs, the nosotros form looks the same in the present and the preterite. So only the context tells you which one is intended. In a process description, it is often understood as a present describing a step.

Why is there no nosotros in the sentence?

Because Spanish usually drops subject pronouns when the verb ending already makes the subject clear.
Alzamos already tells you the subject is we.

So:

Alzamos los cuadernillos... = normal, neutral
Nosotros alzamos los cuadernillos... = possible, but more emphatic

English usually needs we, but Spanish often does not.

What exactly does cuadernillos mean?