Positive start helps to get comfortable and build rapport. There should always be something positive to say about your colleague, even when the harsh negative feedback follows it.
Aim to make the feedback sharing process a positive experience.
However, be careful not to overpraise. You don't want the person to end up thinking, "I'm such a valuable productive colleague." But rather, "Ok, I did a great job designing the concept, but the copywriting part needs way more improvement. Let's work on that."
Note: Do not use "Shit Sandwiches." They don't work. Honestly mention strong and weak points, don't try to create a "shit sandwich."