Ytdyklly

I play classical guitar and usually, I like to play music from movies, video-games or popular songs. However, there are very few classical guitar arrangements for this type of guitar, while there are lots for fingerstyle guitar. I like them a lot, but I also love classical guitar and I would like to play them in classical guitar.

However, I know zero music theory. I used to play the piano and there I learned basic music theory that I don't remember because I never used.
For example, I don't know how to identify the chords in a song. Not even when I see the piano sheet.

How can I start? I read this article, and this seems like a really nice first method to start this adventure.

My plan is the following:

First: I need to find the piano arrangment (something like this link).

Second: Break the song in melody and harmony

Third: Transpose the melody so it can be played in the first strings.

Fourth: Do the same with the chords.

?

Profit

Is this a nice plan to start with?
I don't know how to transpose pieces and also don't know how to identify the harmony in a song.

Also, I plan to keep learning (don't really know what it is) music theory. My ultimate goal is to be able to use the sheet music as a guide for the notes, considering their relative distance to get the melody, using music theory to get a nice harmonization of the piece, and using the techniques that I like when playing the classical guitar.

It would be hard to start if you can't identify chords, so it might be a good plan to start just there. You'd need to build some fluency with being able to play a chord from the sheet in a few different positions on the guitar. This is a basic tool of arranger and it's really worth learning as generic skill useful in many musical situations.

You don't really need to dig deep into music theory around chords - it's all about a skill where you see a chord (for example Cmaj7) and you know a few places it can be played on the neck. It might seem a bit overwhelming at first but it really pays of and gets easier the more chords you know (as you naturally start noticing relations and simplifying this map of chords in your head).

I'd also encourage you to learn existing arrangements even if they're not really tunes you ultimately want to play. You'll learn a lot about how the songs are arranged on guitar, how to combine chords and melodies etc.

It’s a good idea to look for the song in piano style. So you can refresh your piano knowledge.

But if you search not only sheet music when you look for a song you can also give in the title and “chords”, “tabs” or look under “image”. Then you will find the song with the chords.

Under “tabs” you will learn to read the tab notation and you can see which strings you have to play in which bend.

With these 3 systems you can improve your understanding for chords and guitar playing. And when questions will arise you’ll look up the basics of theory.

I love doing simple arrangements for classical guitar, often based on piano sheets (see my profile for examples).

Also, I plan to keep learning (don't really know what it is) music theory.

Delete that "also". To learn music theory should be your priority. Otherwise it's practically hopeless.

You need to be able to read sheets (piano and guitar), to identify notes, harmony (scales, chords), to understand how they are formed - in short, how the tonal music of the "common practice period" works.

It takes some work, but it's invaluable knowledge, and it will also help you to improvise, descipher songs, and enjoy playing in general.

Something that might help, if you already are moderately confortable playing guitar,
is to take simple songs you already know and identify the chords and the scale, and
try to make little changes and ornamental notes.

Also, it helps to use the guitar as accompaniment, strumming chords and singing over it.
Especially if you try to find the chords yourself, find the melodic line on the guitar, transpose it, etc. When you start making progress with theory, you should begin to understand why (for example) a Cm chord is formed the same as Am but with a bar in the third fret, etc.