Practicing and instrument isn't like learning another subject like math or English. It is more like working out. While you can cram the night before for a quiz, you cannot work out a lot the day before a body building competition and expect to win. Getting good at a musical instrument is a gradual process and requires consistent practice. It's much better to practice fifteen minutes a day than two hours on one day a week.
People often built their practice sessions around time of day, but I would encourage you to build them around energy. Practicing correctly is some of the toughest mental exercises that you can do. You should be incredibly focused, which requires energy. While you might have time at the end of the day, you might not have the mental focus to make it a productive session.
If you must have some sessions when you are tired, then make sure you are practicing things that don't require the same level of mental effort. For example, if you are working on your endurance, then putting those exercises at the end of the day would make sense. You will be fatigued, and they don't require much mental engagement to perform.
Do you use a notebook for math class? What about science? English? Chances are you have a notebook for every subject you study. Keeping notes is a good way to remember concepts, set goals, and track your progress. Just like you look at your notes prior to a test, looking through you practice journal can help remind yourself of things your teacher might have said or breakthroughs you learned during your personal practice.
A practice log is different than a journal because it only tracks how much time you spent practicing. This is useful for accountability and for setting practice goals. For example, you might think that you are practicing most days, but when you start keeping a practice log you see that you actually only practice three days a week. Uh oh.
If you notice that there is a day that you always miss, then you can ask yourself "why?" Perhaps you notice that you never practice on Saturdays. When you think about it, you realize that you have football practice in the afternoons and then afterwards you are too tired. When you realize this, you can make sure to move your practicing to right after breakfast. Ta-da, now you can get 15 minutes of practice in on Saturdays.
Or, perhaps you want to make a personal goal to practice three hours ever week. Keeping track of how many minutes you have practiced in a log will let you know when you have achieved your goal. So much of consistent practice is staying motivated and nothing helps motivation like accomplishing a goal.
A practice journal looks at more of the details of your session. In addition to tracking the amount of time you practice, you also track things like your goals for the session, what went well, what didn't go well, and what you plan to practice in your next session.
A basic journal can be made from any old notebook and a practice page should include:
Keeping track of this information will help you discover things like "When is the best time for me to practice; morning or night?" or "I really struggle with playing high notes softly and I have no idea how to fix it." In order to keep track of every detail, it is best to keep your practice journal on the stand or next to you during your sessions. Think of it like a map that tells you where your session should be heading and you are writing down landmarks along the journey. If you wait until after the session, two things might happen: (1) You will forget to write anything down at all. (2) Those "ah-ha!" moments won't be as vivid and you'll forget exactly what you meant or what inspired you.
Here is a free pdf music practice journal for you to print out and use.
One of my favorite ways to keep track of my practice sessions is by using Evernote. It is a free tool that you can access on any of your devices (which means I never forget it or lose it). It also allows you to add files: this could be a pdf of the music you are practicing or a recording of you practicing the excerpt. I can't emphasize how important it is to record yourself playing. Hearing yourself play will let you more accurately evaluate how something sounds. Recording sections can also be useful for your teacher - sometimes you experience something that you want to ask about, but can't explain or recreate it in a lesson.
Full disclosure, Practoid is another project that I am working on and it is a paid tool. It's a music lesson app that helps music teachers and students stay in sync with practicing. The teacher can document the lesson in the student's practice journal and assign digital copies of music from the library. Students can easily see what they need to practice and keep track of their progress in the digital practice journal. The teacher sees all of this, making it easier to prepare for the next lesson and to identify where problem areas.
I like to keep all the music I need to practice in a binder. Usually that includes some warmups, etudes for the week, a solo, and some excerpts. Instead of having a separate journal, I like to keep my notes with the piece of music I am working on. That way, if I can quickly see the detailed notes I made last time. Here is an example of the piece specific journal entry page. I put this right behind the music in the journal.
You can see I break down the components of the selection into minute detail. What I am looking for is to make each category perfect. I will ask myself "What could make my tone more perfect? Is it the right sound at the right dynamic on every single note? If note, which note and what can I do to make it better?" From there, I know what to practice next time.