Learn Spanish (Latin America): Introducir el hilo en la bobina nueva.

This is an impersonal instruction, a very common structure in manuals, recipes, and signs. Using the infinitive makes it a general step for anyone reading the guide, rather than a direct, personal command to a specific individual.

Wait, doesn't introducir mean "to introduce"?

It's a classic false friend! Introducir means "to insert" or "to put into" (like a thread, a coin, or a password). While it can mean "to introduce" a new concept or product to a market, if you want to introduce a person to someone else, you must use presentar.

Why is the preposition en used for "into"? Could I use dentro de?

In Spanish, en is heavily multi-purpose and covers "in," "on," "at," and "into." It's the most natural choice after verbs of placing or inserting. You could use dentro de if you really wanted to emphasize putting it "inside of" something, but en is the standard, concise choice for manuals and instructions.

Could we put the adjective before the noun and say la nueva bobina?

You could, but it changes the nuance. Placing nueva after the noun (la bobina nueva) emphasizes that the bobbin is physically brand new. If you put it before the noun (la nueva bobina), it means "the newly acquired bobbin" or "the replacement bobbin," which might actually be a used part. In technical instructions, physical descriptions usually go after the noun.