A field journal gives a short walk a lasting shape. Rather than collecting every possible fact, it preserves a route, a handful of observations and the questions those observations raise. A single page is enough for a useful beginning.
Write the date and starting point, then choose a limited focus such as surfaces, signs or gathering spaces. Work from public areas and respect photography restrictions. Make notes while stopped somewhere suitable so the walk remains attentive to its surroundings.
Use separate marks for what you observed, what someone told you and what you want to verify. This small distinction becomes valuable later, when an appealing explanation might otherwise be remembered as something you actually saw.
At home, select one photograph or sketch to accompany the entry. Add context without filling gaps from imagination. A good field journal remains personal while making the basis of its statements clear enough for another reader to understand.