This is an important task that can help to move on new features but also bugfixes, and you can contribute very easily!
It is required a dev environment, with git, so an online environment is not very good.
Personally I suggest VVV but you can have your docker or other solution.
For VVV I want to remember that there is the support for ClassicPress https://github.com/ClassicPress/classicpress-template-develop
Before asking what is VVV, I suggest checking those links and try it and open a new thread if you have any issue or reach me on Slack.
How to test a pull request
The most simple way is to install hub a tool that extend git natively.
I suggest it because simplify a lot the issue of download a fork, checkout to the branch and keep update it that branch.
Of course you need a ClassicPress instance working with git!
git clone https://github.com/ClassicPress/ClassicPress/
Every pull request is a git branch on a fork but with hub is very simple to download and access as local branch!
cd ClassicPress #Enter the folder where is ClassicPress git checkout https://github.com/ClassicPress/ClassicPress/pull/428
Automatically you will be in the new branch that have the name of the number of the pull request.
You can update the branch (so the pull request locally) easily with
git pull .
This is a quickie guide, for any other improvements let me know and I can add more details if something is not clear.
I just written because of the enthusiasm on Slack about it