Loop Posts into Varied Width Divs

I’m not understanding how to get the loop to feed posts into my layout. I’ve coded the html and css (php is a challenge) but after trying examples in the codex and forum, I am lost. I understand html and (I think) I understand the loop but the bridge between the two is not coming together.

Goal:
Row 1: 100% width
Row 2: one 75% and one 25% width
Row 3: two 50% width
Row 4: three 33% width

Any help with the most bare bones noobie example of how the loop and html interact to
get this to work would be appreciated.