It happens more than it should: I walk into a lesson and the student is jaw-droppingly unprepared and the excuse offered is, “I didn’t have enough to practice.”

It was a frequent occurrence in my own early lessons. I would pour hours into my practice with only marginal progress, eventually decreasing my practice time to the bare minimum my parents would allow, feeling utterly defeated. I was able to skate along for a little while, but things got unpleasant when the repertoire on the stand outpaced my natural ability. 
It’s not that I didn’t have enough to practice. It’s that I had no idea what to practice. How to practice. What I find is that how and what are actually interchangeable. Here’s what I mean. 
Let’s say you’re working on a piece. First off, you know you need to break it up into sections. Most adults instinctively do this, but just in case there are any younger/newer students reading this, I’ll restate that. I liken it to combing knots out of long hair. When there’s a snarl in the middle of the strand, you don’t continue running the brush the entire length of the hair: you do a thousand small strokes at the site of the tangle until it’s gone. Then you gradually take longer and longer strokes until you really can get the brush through the entire length of the hair. Same thing here. You play until there’s a problem, and then work the problem. Then you back up a few measures and see if you’ve really untangled it. 
So you’re working in sections. You notice two nasty sounding elements: a quick run of notes that leaves your fingers dizzy, and string crossings that are uneven or maybe a little percussive (aka crunchy, scratchy, noisy). 
If you don’t notice anything particularly wrong, it means you’re not listening with the right ears. Record yourself and compare that recording to a professional one. Then do the math: what are the differences? Don’t go to the negative place (I’m horrible, this is impossible), instead go to the grittier, tougher place. Every pro used to have gnarly shifts and ugly string crossings. Then they were honest with themselves about how to fix it, and set about doing those things. If you do the same, that will be yet another aspect of your approach you have in common with the big dogs. And that’s what we all want. 
Oh, and if you don’t have the ability to record yourself, read your teacher’s notes and assume they’re accurate. There should be phrases in there like:
“keep the bow in X part of the string”
“rotate your hand X”
“work on X” 
And if there aren’t, I have two pieces of advice. 
1. Ask for technical advice like that in your next lesson. 
2. Just assume that every element of your technique needs work, pick one, and get cracking. 
Ok, so back to the tricky run and the bockety bow. My whole point is that how to practice and what to practice are the same things. 
So for the run, you need to extract what’s tough about it. Usually the answer is something like:
1. It’s a little too fast.
2. The notes don’t fit well in my hand.
3. It contains new technique (extensions, thumb position, &c.).
4. My bow and my left hand are out of sync (see #1).
So how to practice? Well, saying, “I want this to be better and faster” is like saying, “I want to have an enchanted tree that grows money!” Sure. But nobody ever got rich from a magic tree, and nobody ever became an expert at the cello by wishing for the end result. What you have to do is train your sights on the fix. If something feels too fast, slow it down. More than you want to. WIth a metronome. And then repeat it more than you think you should. You’ll know you’re in the productive zone when you think “Surely I have it down now!” and then repeat it 20 more times at the same speed. The what is the how. I said this a lot on my last teaching tour, I say it in most of my lessons, and I’ll bang on it until it’s dead here: in practice, have a physical goal. Examples of physical goals during this passage might be:
1. I will take short, steady bows.
2. I will prepare my left hand for the next note.
3. I will focus on keeping my thumb “tripod”-ed behind my fingers. 
4. I will concentrate on my breathing until I can breathe and play at the same time. 
So now, it’s your turn. You have in front of you something tricky for the bow. Let’s say you’re looking at our Darling: Prélude, Suite 1. No matter how you bow it, it’s easy to get pushed out of shape and away from a musical place. But just for the sake of uniformity, I’ll say you’re going to try to play it 8 notes to a bow for the first few measures. 
In the comments section, tell me what the difficulties might be, and then offer some physical goals designed to confront and fix those things. Just like it helps to hear a phrase in your head before playing it, having an idea of what good practice looks like is useful when you get stuck. 
Returning to the implicitly accusatory, “I didn’t have enough to practice”, for a moment, there have been times when I have responded silently. I take the current étude book, open it to any page at all, point to a line, and motion to the student that they should play it. 
Afterward, I crack the smallest smile and say, “There is always more to practice.” 
What they don’t see is how this little vignette inspires my own ethic. 9 times out of 10, I return home, open Popper or Bukinik to a random page, and get to work. There is, after all, always more to practice.

Cool photo from iLeadScaffolding.