A scene can only be examined once. Every footstep, every opened door and every moved cup changes it. So the first job is not collecting, it is protecting and recording.

1. Secure. Limit who comes in. A logbook lists everyone who enters, with the time. Often a single marked path (with stepping plates) is used, so people do not walk over traces on the floor.
2. Dress to avoid adding your own traces. Gloves, shoe covers, a hair cover and a mask keep the examiner's fibres, hairs and prints out of the evidence.
3. Record in three ways: notes, photographs and a sketch.