In my years as a young classical guitarist I cannot count how many times I took on a new piece of music and quickly hit the notorious "brick wall". Every piece no matter how big or small, poses various issues, but if you have a well developed game plan, any piece can perfected in a reasonable amount of time.
The Game Plan.
1) Breaking things down.
The first thing I do with a new piece of music is to spend some time simply looking it over, without my guitar. I like to read though the music and read it as if it were a story, thinking through phrasing, fingerings, positioning, etc. I do my best to hear the music in my mind and think about how I may attempt to execute shifts, bring out melody notes in chords, balance counterpoint, etc.
After I spent some time looking over the music and feel somewhat comfortable with it, I will then begin to play through parts of the music on the guitar. As I am playing through the music I focus on breaking the piece into small sections, sometimes as small as a phrase or as long as a full page. By breaking the music into small sections, it makes it much easier for me to focus on perfecting technical and musical issues without being overwhelmed.
2) Time for the Pencil!
The most valuable tool you can have on your music stand when working through a new piece of music is a pencil. One of my favorite things to do in my lessons is to show my students music that I have worked on so that they can see just how much writing I do throughout the pages in order for me to better play the piece. It can be as simple as circling a fingering or rewriting a fingering, to translating a musical term to get a better understanding of how the composer wants the music to be performed. My favorite example to show to my students is my copy of Joaquin Rodrigo's "Fandango", where above a particular phrase with a tricky fingering I wrote a note to myself, "use this fingering you idiot", which I wrote to myself because of a poor fingering I was attempting to use, causing the phrase to sound choppy.
Now, as a disclaimer, I have friends and fellow teachers who disagree with me on this point, but I find it to be very beneficial to listen to the piece that you are working on. One advantage our generation, and future generations of guitarists, have is that we have easy access to recordings and videos of a lot of classical guitar music. It is very simple for a person to log onto the internet and find videos on youtube of many different people performing both formally or informally (in their bedrooms or on the concert stage), or to download a recording.
I encourage my students to do this as much as possible, and also give them "listening assignments" where they need to find a recording/video of works by various composers or performances by various guitarists. I understand that there are some dangers in this however, as a student could see a poor performance of a piece of music and think that the music should sound that way, so in order to help send them in the right direction, I provide my students with a list of high quality, professional guitarists to look for. I feel that by listening to piece of music, and many different performances of that work by different guitarists, not only helps develop a connection to that piece of music, but also can help develop a sense of personal taste and style. For example, I like to play recordings of J.S. Bach's "Bourree in E minor" for my students, and chose 4 different guitarist (Julian Bream, John Williams, Paul Galbraith, and Andres Segovia). I ask my students to describe the differences in their interpretations, what they liked and did not like, and what they would do differently, etc.
I make sure to explain to my students that although it is a good idea to listen to a recording of the piece they are working on, it is important to eventually come up with their own musical ideas for the piece, and not to copy exactly what the performer on the recording has done.
In conclusion, I hope my "Game plan" helps you with your attempts to start that new piece of music. Remember, read through your music to become familiar with it, break it into sections so that you can focus on smaller details, mark your score (in pencil!), and if you choose to, listen to high quality recordings or watch high quality videos of you music so that you can keep the sound in your mind.