Many people skip their first entry because they feel they need a name for the problem before they write anything down. That is a mistake. A useful log starts with a plain sentence such as "I don't know what this is yet." Your log is a record of what you saw, felt, and did. It is not a diagnosis, and it does not need one to be worth keeping.
Write what happened, not what it might be
A name for an injury usually comes from a trained examiner, after questions and sometimes other checks. Until then, the job of your entry is smaller: describe the event and the body part in words that anyone could check. "Stepped off a curb and my right ankle rolled" is a complete entry. "Probably a sprain" is a guess, and guesses look worse over time when they sit in a record for weeks.
For your first entry, cover four things: what you were doing, where it happened, what you felt in the moment, and what you noticed in the next hour. Keep each part to a sentence or two.
Separate what you observed from what you think it means
Think of two columns, even if you only write in one. Observations are things you could point to or measure: swelling you can see, a spot that feels warm, a movement you could demonstrate, a time you first tried to walk on it. Interpretations are what you believe the problem is. Both have a place, but mixing them makes the record harder to trust, including for you when you read it back later.
When you are unsure, mark the line as unsure. "Felt like it popped, not certain" is honest and useful. It tells a future reader exactly how much weight to give the detail.
Put a time on the first line
Every entry gets a date, and a time when you can manage it. Approximate times are acceptable if you label them. "Around 6 in the evening, based on when the game ended" is far more useful than a blank line. Dates let you see whether something changed, stayed the same, or moved to a new area over several days. That pattern is often the information a provider asks about first.
Keep the first entry short enough that you will write the second one. A few clear sentences on day one are better than a detailed page you never repeat.
A starter checklist you can copy
Use these prompts as a guide, not a form you must finish. Write the date and approximate time. Describe the activity at the moment of the event in one plain sentence. Name the area of the body you noticed something in, using your own words. Note what you felt, such as sharp, dull, hot, numb, or tight, and whether it was constant or came and went. Record whether you could bear weight, lift, grip, or move the area as usual, answering yes, no, or partly. Finish with anything you did about it afterward.
When the entry should push you toward a visit
A log supports a professional look. It does not replace one. If the area looks deformed, you cannot use it the way you normally would, numbness or tingling is spreading, you hit your head, you have trouble breathing, or symptoms feel severe or are getting worse, do not wait to finish your notes. Seek prompt care right away.
If you are in the St. Louis metro and deciding whether to get examined this week, write the first entry now, then read it aloud before you call a clinic or go in. It helps you describe what happened in the order it happened, with the times you wrote down, instead of trying to reconstruct it from memory under pressure.
Keep the entry plain
Dated, specific, and boring is the most useful kind of entry. Skip exclamation marks, guesses about cause, and comparisons to other people's injuries. Your ledger is for you and for whoever helps you next. Each entry gets a date, and each date gets a plain sentence. That is enough to begin.