The order is the advice. Each step is better than the one after it.
Run. If there is a safe route, take it — that is better than surrendering the choice to somebody else. Leave belongings. Do not gather your things, do not wait for others to decide, and do not stop to film. Insist others go with you, but do not let their hesitation stop you. Stop people from entering as you leave.
Hide. If you cannot run: get out of sight of doors and windows and put something substantial between you and the danger — a brick wall rather than a plasterboard partition. Lock and barricade. Silence your phone completely, including vibration. Move away from the door. Stay down and stay quiet.
Tell. Call the emergency number when it is safe to speak — 112 across the European Union and the EEA, the local emergency number elsewhere. What they need: where you are, where the attacker is, how many, what they look like, what weapons, which direction they went, how many casualties.
When armed officers arrive.
- Keep your hands visible and empty. Do not make sudden movements towards them, and do not run at them or hold onto them.
- Expect to be shouted at and treated as a potential threat until you are not. That is procedure, not suspicion.
- They may go past injured people. Their first job is to stop the attack; medical teams follow behind and cannot enter until they do.
- Do as you are told, immediately, and evacuate the way you are pointed.
Afterwards. Do not post locations of people still hiding, or of police positions. A live broadcast of where people are sheltering is information for the attacker.
Source. ProtectUK, the public advice service of UK Counter Terrorism Policing, Run Hide Tell, https://www.protectuk.police.uk/advice-and-guidance/response/run-hide-tell
What this is not. Reading, not training, and not the procedure of any police service. Follow the instructions of the emergency services where you are.