Can I just describe what I did instead of filling in a form?

Yes. Describe it turns a few sentences about what you've been doing into entries you can check and save.

How to use it

Open the log sheet with Log on Home or in the menu (or the + on a phone), and choose the Describe it tab. The Log page also has a Describe it button at the top.
In the box ("Since 2:15 PM, in your own words"), write what you did since your last entry. Or press Speak to dictate, if your browser supports it.
Press Draft entries. You get one draft per thing you did, each with its end time, project, activity, people, energy and notes filled in. A time the app had to guess is marked time guessed — check it.
Fix anything that's off, then press Save on a draft, or Save all to save them together. ← Change what I said takes you back to your words.

Nothing is saved until you press Save. If you close the sheet, your words and drafts are still there when you come back.

What it can't do

It only covers the time since your last entry, so you need at least one entry first. For an earlier gap, see I forgot to log part of my day.
It takes up to 4,000 characters at a time.
If you logged something on another device in the meantime, saving is refused instead of shifting your times. Draft again.

Questions

I don't see Describe it. It isn't available on every account. The Log form is always there.

Where do my words go when I dictate? Speak uses your browser's own speech recognition. In Chrome, that means your audio is processed by Google.