When you work with any programmer, it's good to give an introduction and checklist of things to do before the code is written. Reviews and corrections are often expensive and time-consuming, waste time and energy for many in the group.
Some important aspects are
All the points above talks more about writing better code than struggling to give feedback on poorly written code.
There are a lot of learnings can be derived from open source community where senior and newbies are equally participating and contributing to the project.
First, to have an up to date "README" page, which contains all essential things to take care of in the project. GitHub and similar services render README files at root level beautifully as a web page. A README can help the people to understand how they can up and running in minutes. More: Readme based development
It's equally important to adopt any branching workflows such as GitFlow, GitHub Flow to ensure the code is written, tested and incorporated in a structured way. This also helps to have a pull request ready for review. All these information can be part of the README file as appropriate.
Having a suitable PULL_REQUEST template will help show a template, which contains checkpoints and other information to be checked and filled before raising a PR.