How to practice is a large subject. There should be a routine of scales, etudes and technical development. For now, I would like to discuss how to practice and prepare for upcoming ensemble rehearsals and a concert.
In school music groups that meet daily, students are presented new music and the norm is not to practice the music before rehearsals begin. The teacher is leading a practice session and skill development along with learning the music. In a professional ensemble, the musicians are generally expected to know their own parts before rehearsals begin. Youth symphonies and community ensemble members that rehearse only once a week should try and follow the professional model as best they can.
First, get a hold of the music. There are free sheet music internet sites such as the International Music Score Library Project (IMSLP) for much standard literature. A smart music director will make practice parts easily available. Next, if the music is new to you, listen to a recording. YouTube is a great source for this. You can often see and hear people performing your music. Having an idea of the whole piece of music and how your part fits in will help you with setting practice goals like tempo and style.
In practicing, goals need to be set, these should be short-term and long-term. Sometimes for a highly skilled player the music is simple enough that they might be able to sight read it. If the music is challenging and new to you do not expect mastery in one practice session. Most all good learning is incremental and will take time. Do not expect mastery on the first practice session. Make a plan over time, do not get discouraged if you do not get it right the first time. Often you will find the second, third or fourth practice session over certain music the most rewarding.
Learn to vary the musical elements to work out difficult sections of the music. Tempo can be slowed downed greatly and rhythm can be altered for special sections of the music. Find a steady tempo you can be “goof proof” on, meaning no mistakes at that speed. Then gradually increase the tempo. You cannot easily vary pitch in your practice, however if it’s a difficult shift for string players sometimes a practice a slide or glissando can help you learn to measure the distance. Tone should always be your best. Varying that will not help, you want to hear yourself well. Practice soft only when it is in the music.
Varying musical style can be very helpful in practice. A difficult pizzicato section might be played with the bow to fix pitch and a bowed section can be played pizzicato to focus on left hand. Wind players and string players can vary staccato or legato practice to work out sections. If the music is spiccato (bounce bow for string players) play it on the string until you can handle the tempo.
Put in some time with the music before the rehearsal. You will get more out of the experience, help the ensemble and look forward even more to the next rehearsal and the concert.