How do I keep the site journal correctly?

Short answer

Fill in the journal on every working day, not retroactively at the end of the week.
Record the crews present, the headcount, the equipment used and the stages worked on.
Note the weather and any stoppage, together with its cause.
Attach dated photos of anything that will be covered up: foundations, reinforcement, service routes.
Record separately the events that belong in the construction technical logbook — site instructions, key inspection milestones, incidents.

Law 10/1995 on construction quality — the construction technical logbook is compiled during execution and kept for the entire life of the building.
The construction acceptance regulation (Government Decision 273/1994, as amended by Government Decision 343/2017) — the journal of events forms part of the technical logbook, alongside the design, execution and acceptance documentation.
Mind the distinction: the daily site journal is the contractor's operational record, while the journal of events in the technical logbook only records events relevant across the building's life.
Government Decision 300/2006 on temporary or mobile construction sites — the health and safety coordinator keeps the coordination register, a document separate from the site journal.

Example: for the ground-floor slab pour, the journal for 12 May records a crew of 8 carpenters, one concrete mixer truck, 42 m³ of concrete placed, 9°C, and the time the pour started. That same day has 6 photos of the reinforcement before the pour — the only evidence of how it was built, once concrete covered it.
Example: four rain days that stopped exterior finishing, recorded with their cause and notified to the client, later support a request to extend the contractual deadline.

Filling in the journal once a week from memory — the details that matter in a dispute are the first to go.
Not photographing work that gets covered up; after the pour there is no evidence of the reinforcement left.
Not recording stoppage days and their cause, then being unable to justify the delay to the client.
Confusing the daily journal with the technical logbook's journal of events, and ending up with neither one complete.

The journal is filled in from a phone or tablet on site, offline included, and syncs once back online.
Photos stay tied to the day and the stage they were taken for, not in a separate folder.
Subcontractor work statements and client progress reports are generated from the journal automatically.
Working days with no entry are flagged, so you don't discover the gaps at handover.