We used php templates to split the page into sections and each took different parts. Using Sass, we started by creating a file of common variables we would both need (such as colours, design breakpoints etc.) and adding more as they came up.
Git was used for version control and so that we could work remotely.
Things To Improve:
For the most part this went really smoothly being one of my first collaborations. We went from desktop to mobile though which was a mistake and made the CSS messier than it needed to be. Due to this, in future projects I went from small to large with my media queries instead with better results.