Some sections of a piece are easy to play from the beginning, some are difficult, some may even seem impossible. There is hope :)
1. Play the problem passage through so slowly that you can play it perfectly. Stop playing and tap the beat. Turn on the metronome (YES...the metronome!!) and find that speed.
2. Play the passage WITH the metronome one time, perfectly. LISTEN to the beat of the metronome!
3. Raise the speed one notch (for example, from 60 to 63) and play again. (If you have a digital metronome, you can raise the speed by three or four numbers - I prefer a digital metronome because it stays accurate unlike mechanical metronomes which can get off beat and need to be "tuned" )
4. Each time you play perfectly, raise the speed one notch. If you start to feel anxious or if you make a mistake, it's time to stop.
5. Write the day's best score in your music or in your assignment book or on a separate sheet of paper.
6. The next day, start over at step 2 at the previous day's final speed. Sometimes yesterday's speed will feel too fast. If it does, set the metronome back a notch or two and start from there instead.
A week's scores might look like this:
quarter note = 60/72/76/84/84/88
Notice that your speed doesn't always increase from one day to the next. That is all right, and it does not mean that you won't be able to raise the speed the next day. Increases are usually greater at the beginning of the week and smaller as the days go by.
Doing this drill a few times is not enough, even if you reach the correct tempo for the piece. To have a lasting effect, it should be repeated daily until the passage is thoroughly comfortable at or even above the desired tempo. This process could take days or, with an advanced piece, even weeks.