Anyway, in the comments, people mentioned that one of the hardest aspects of making a to-do list is estimating how long activities take. I agree that this is hard. Indeed, the ability to estimate how long things take is what separates time masters from other people. However, I also think, as with most skills, that almost anyone can reach competence with practice. Here's how to estimate like a pro:
Track your time. This may seem obvious, but one way to figure out how long things take is to actually record how long they take. This is not a perfect solution — as it's new or variable tasks that usually flummox people — but it is a reasonable one. It particularly helps for people who suffer from the problem of wild optimism. These people think it takes 30 minutes to get to work, because it did — once — but now that one random time becomes the baseline for a trip that normally takes closer to 45 minutes. Which explains why they're always 15 minutes late to their first meeting. It becomes harder to think of a trip as taking 30 minutes when there — in black and white — it keeps taking 45 minutes, five work days in a row.
(I have the opposite problem of profound pessimism. But that is a different matter.)
Break it down. Big, nebulous tasks can be hard to estimate. Do your taxes? Hmmm. But going through 12 months of your business credit card statements and entering those numbers in your spreadsheet might, reasonably, take 10 minutes per month. At 12 months, that's 120 minutes, or two hours. So you can build that component into your estimate. When I propose my books, I have almost always come up with chapter abstracts. So I know how many chapters there will be. I can figure out how much time to devote to each chapter (often, a 6,000-word chapter a week - 1500 words per day plus an edit day). So a 10-chapter book would take 10 weeks to produce a draft.
Do check-ins to tune up your estimates. If, for instance, you thought you could write a chapter a week, but on Friday of week one you are no where near the end of the first chapter, your estimate is off. Time to revise with the more realistic time line in mind. Fun fact: while most things take longer than expected, this isn't always true. Sometimes your time line can revise to be quicker. While reading War & Peace, for instance, I thought I would read through 4 percent per day on my Kindle (approximately 1 hour), thus completing the book in 25 days. I quickly saw that I was hitting 7 percent per day without much trouble. So I was OK with estimating I'd be done in 2 weeks instead of 3.5.
Ask other people. Speaking of estimates…That War and Peace estimate is courtesy the Kindle app, which analyzes how long various readers take to read a given book. I could have estimated that I read about 50 pages an hour, and War and Peace was about 1450 pages, giving me 29 hours, but Kindle understands that Tolstoy's chapters are short. There's white space in those page counts. Crowdsourcing answers is a great way to figure out how long something you've never done before will take. In the old days, if you were landing at the San Francisco airport, and then you were driving to Oakland, you would have asked someone you know in the area "hey, how long does it take to get from SFO to Oakland?" Now, of course, you can crowdsource via various apps that have analyzed thousands of drives. But it's really the same thing. Ask other people how long they think this thing you've never done before will take. Come up with a rough average of their answers, or at least get a probable range.
Account for your energy. The hard truth is that tasks take longer when you have limited energy to do them. You'll get distracted, and a report that might have taken an hour to write at 8 a.m. takes 2 hours at 2 p.m. as you keep clicking over to your inbox (or even less work-related things). If you've got to schedule it in at 2 p.m., fine, but be honest with yourself. It might help to think in terms of probabilities: Memos like this take me 45 minutes to an hour, most of the time. If you've blocked it for a low-energy time, build in an hour. If you're hopped up on your 8 a.m. coffee? 45 minutes. 40 if it was a double shot of espresso.
Build in space. Given that optimism is the more common curse with time estimation, most people would do best to engage in "harm reduction": that is, underestimate how much you can get done in any given day (or even week). If you're wrong and you do more, or you finish early — awesome! You've got extra free time. Not a problem. If you overestimate, you're behind. And that is generally a much bigger problem.
As a practical rule-of-thumb for building in space, I would recommend not booking more than 75 percent of your day ahead of time.* So let's say you would like to be at the office for 9 hours (8 hours of work, and 1 hour of lunch/coffee/internet/bathroom breaks). If you have 3 hours of meetings, you can assign yourself additional tasks that total 3 hours. No more. That gives you 6 hours of planned activities during 8 hours allocated to work.
The reason for this is that many things take longer than planned. One of those 1-hour meetings becomes a 90-minute gab fest. Or stuff comes up. Occasionally good stuff! If you've assigned yourself 5 hours of activities + your 3 hours of meetings, and a former client calls to talk about work you might do together, what are you going to do? I'm guessing you're going to take that call. And so something will get moved. It might be personal time (you stay late). It might be a less-important meeting that gets rescheduled (Ok, but can create schedule chaos for others). It most likely will be one of your other assigned activities — but what if those were really important? Like coming up with a proposal for another client who called you earlier? This is how people start to feel frenzied and behind. With better time estimating skills, this feeling becomes less likely.
*I know this is not practical in all fields. A medical office might aim to book the entire day. Of course, this is why appointments often start late. Building in a bit of space can get the schedule back on time.