Je ne trouve plus la notice, alors je me demande comment brancher la multiprise correctement.

In French, ne ... plus is a complete negative expression meaning no longer / not anymore.

So:

• Je trouve = I find / I am finding
• Je ne trouve plus = I can’t find it anymore / I no longer find it

You do not add pas here.
French commonly uses special negative pairs:

• ne ... pas = not
• ne ... plus = no longer / not anymore
• ne ... jamais = never
• ne ... rien = nothing
• ne ... personne = nobody

So Je ne trouve plus la notice means the speaker used to have it or expected to have it, but now cannot find it.

French uses articles much more often than English does. In this sentence:

• la notice = the instructions / the manual
• la multiprise = the power strip / extension block

Even where English might sometimes say I can’t find instructions, French usually prefers an article: Je ne trouve plus la notice.

Here, la suggests a specific manual and a specific power strip that both speaker and listener can identify from context.

No. French une notice is a leaflet of instructions, manual, or instruction sheet.

It is a false friend for English speakers, because English notice usually means:

• a sign
• an announcement
• a warning
• advance information

So in this sentence, la notice is the instructions that came with the device.

Because demander and se demander mean different things.

• demander = to ask
• se demander = to wonder / to ask oneself

So:

• Je demande comment brancher la multiprise would sound like I ask how to plug in the power strip
• Je me demande comment brancher la multiprise means I wonder how to plug in the power strip

The reflexive pronoun me shows that the speaker is asking the question internally.

After comment, French often uses the infinitive when the subject is understood and stays the same.

So:

• je me demande comment brancher la multiprise = I wonder how to plug in the power strip

This is very natural in French. It works like English how to plug in...

You could also have a full clause, for example:

• Je me demande comment je dois brancher la multiprise = I wonder how I should plug in the power strip

But the infinitive version is shorter and very common.

French adverb placement is flexible, but with an infinitive like brancher, putting correctement after the object sounds natural and clear here:

• brancher la multiprise correctement

This means to plug in the power strip correctly / properly

You may also sometimes hear:

• comment bien brancher la multiprise

Here bien is more common in everyday speech, while correctement can sound a little more formal or careful.

Alors here means something like:

• so
• therefore
• in that case

It links the two ideas:

1. I can’t find the instructions anymore
2. so I’m wondering how to plug in the power strip correctly

It is a very common connector in spoken and written French.

Because the point is not simply I do not find the manual, but I don’t find it anymore / I can’t find it now.

Compare:

• Je ne trouve pas la notice = I can’t find the manual
• Je ne trouve plus la notice = I can’t find the manual anymore

Plus adds the idea of a change from an earlier situation.

Yes, it is natural French.

brancher means to connect, to plug in, or to hook up, depending on context.

Examples:

• brancher la télévision
• brancher le chargeur
• brancher la multiprise

In context, brancher la multiprise means connecting the power strip to the electrical supply properly.

Yes. Brancher is broader than just inserting a plug into a socket.

Depending on context, it can mean:

• plug in
• connect
• hook up
• wire up

So comment brancher la multiprise could refer to the practical setup of the power strip, not just the single action of pushing a plug into the wall.

Because comment directly introduces the infinitive clause.

French says:

• Je me demande comment faire = I wonder how to do it
• Il explique comment utiliser l’appareil = He explains how to use the device

You do not say comment de brancher.

English speakers sometimes expect an extra word because of phrases like a way to or instructions for, but after comment, French goes straight to the infinitive.

In ne ... plus meaning no longer / not anymore, the final s is usually not pronounced:

• Je ne trouve plus → roughly plu

That is the standard pronunciation in this negative use.

By contrast, in the mathematical or additive sense plus meaning more / plus, the final s may be pronounced, especially before a pause or in careful speech.

So here, learners should expect plus to sound like plu.

Yes, and it would sound natural, though it is slightly different in nuance.

• Je ne trouve plus la notice = I can’t find the instructions anymore
• Je n’arrive plus à trouver la notice = I can’t manage to find the instructions anymore

The second version emphasizes difficulty or repeated unsuccessful effort.
The original sentence is simpler and more direct.

Because once the subject is already clear from je me demande, French can leave the next verb in the infinitive.

This is very common after verbs of wondering, knowing, explaining, and showing:

• Je sais quoi faire
• Il montre comment utiliser l’appareil
• Nous devons décider où aller

Using the infinitive avoids repetition and sounds natural.

Both are possible, but they feel slightly different.

• correctement = correctly, properly
• bien = well, properly

In everyday spoken French, many speakers might say:

• je me demande comment bien brancher la multiprise

That often sounds a bit more conversational.

The original correctement is still perfectly good French, and it may sound a little more precise or careful, which fits a sentence about following instructions.