The biggest risk is over-engineering it
A log with too many fields, or one that demands a long written entry every week, tends to get abandoned within a month. The single most important quality of a log is that it actually keeps getting filled in, which usually means keeping it short.
A minimal version that still works
Date, dose, site, weight. Four fields. Everything else (notes on side effects, energy, anything unusual) is optional and can be added only in weeks where there’s actually something worth noting.
Attaching it to an existing habit
Logging right after the injection itself, rather than “later,” is the most reliable way to make it stick, since it attaches the log entry to something you’re already doing every week rather than adding a separate task to remember.
Choosing a format that fits how you actually work
A notebook, a phone note, or a simple spreadsheet all work equally well; the best format is whichever one you’ll actually keep open or nearby at the moment you’re injecting, not whichever looks most organised in theory.
What to do after a missed week
A missed entry isn’t worth abandoning the whole log over. Picking back up the following week, leaving the gap visible rather than trying to reconstruct it from memory, keeps the record honest and keeps the habit going.