Principles only. What makes material admissible is decided by the law of each jurisdiction and by the procedure of each force. This association is not a police force and cannot define either. What follows is how not to destroy the value of something before that question is even reached.
Capture it when you see it. Web pages are edited and deleted. A finding you can no longer show is a finding you no longer have.
Record how you got it. The address, the exact time with its time zone, and what you did to reach it. "Found on their website" is not a record.
Do not alter the original. Keep the file exactly as retrieved. Crop, annotate and highlight on a copy, and keep both. An annotated screenshot with no unannotated original behind it is somebody's interpretation.
Record its fingerprint. A hash of the file, written down at the moment of capture, is what later shows nobody edited it.
Keep the order. Who held it, when, and what they did with it. The moment that record has a gap, the material is arguable.
Do these and the material may still be inadmissible for reasons that are somebody else's to decide. Skip them and it certainly will be.