Ponemos el tejuelo sobre el lomo.

Ponemos is the 1st person plural present indicative of poner.

So grammatically it means:

• we put
• we are putting

In instructional Spanish, this form is also often used to describe a step in a process, so in English it may sound more natural as we place or simply put.

Spanish often uses the present tense to give instructions, especially in:

• manuals
• recipes
• demonstrations
• classroom explanations

So Ponemos el tejuelo sobre el lomo can function like an instruction step, even though it is not an imperative.

Compare:

• Ponemos... = instructional present, common in explanations
• Poned... = you all put... (informal plural command, common in Spain)
• Pongan... = formal/plural command
• Pongamos... = let’s put...

Tejuelo is a fairly specific word. In Spain, it often refers to a small label, especially the kind attached to the spine of a book in a library or archive.

So it is not as general as etiqueta.
A native speaker might use etiqueta in more everyday situations, but tejuelo is the more precise term in library or book-related contexts.

Here lomo means the spine of a book.

Literally, lomo can also mean:

• the back of an animal
• a loin or cut of meat
• the back/spine of an object

In book vocabulary, el lomo is the normal word for the spine.

Yes, both can be possible, but the nuance is slightly different.

• sobre el lomo = on the spine / onto the spine / over the spine
• en el lomo = on the spine or attached to the spine

In this sentence, sobre emphasizes the placement of the label onto that surface.
In real usage, en el lomo is also very natural when talking about something fixed or located there.

So sobre is not wrong at all, but en el lomo could also sound natural depending on context.

Spanish uses the definite article more often than English.

Here, el tejuelo and el lomo refer to specific things already understood in the situation:

• the label we are using
• the spine of the book

English sometimes keeps the, and sometimes it sounds more natural without repeating it so explicitly. But in Spanish, the articles are completely normal here.

Yes, somewhat.

The basic order here is very standard:

• Ponemos
  • el tejuelo
    • sobre el lomo

That is:

• verb
• direct object
• prepositional phrase

You could change the order for emphasis, for example:

• Sobre el lomo ponemos el tejuelo

But the original sentence is the most neutral and natural version.

In standard Peninsular Spanish, tejuelo is pronounced approximately:

te-HWE-lo

More exactly: te-ˈxwe-lo

A few useful points:

• the j sounds like a strong throaty sound, not like English j
• ue forms one syllable here: hwe
• the stress falls on jue: te-JUE-lo

So the syllables are:

• te
• jue
• lo

Poner is very common and natural. It is a general verb meaning to put or to place.

But in more formal or technical language, you might also hear:

• colocar = to place
• pegar = to stick, if the label is adhesive
• fijar = to fix/attach, in more technical contexts

So:

• Ponemos el tejuelo sobre el lomo = perfectly natural
• Colocamos el tejuelo en el lomo = slightly more formal or deliberate

Yes, but it sounds a bit specialized because of tejuelo.

The structure is completely normal, but the vocabulary suggests a context like:

• a library
• book cataloguing
• bookbinding
• archival work
• classroom instructions about labelling books

In more everyday speech, someone might choose a more common word such as etiqueta, depending on what exactly they mean.