Open https://github.com/lightninglabs/lightning-dev-site and while logged in, select the "Fork" option and follow any prompts.
Open terminal and type
git clone https://github.com/YOUR-USERNAME/lightning-dev-site.git lightning-dev-site-yourname
or if you have ssh keys setup for your git profile
git clone git@github.com:YOUR-USERNAME/lightning-dev-site.git lightning-dev-site-yourname
then change to the directory
cd lightning-dev-site-yourname
If there have been commits since you cloned your fork, you will need to update from the master tree.
git remote add upstream git://github.com/lightninglabs/lightning-dev-site.git
git fetch upstream
git pull upstream master
or
git merge upstream/master
Edit the various files in your desired editor. Save and exit.
To add a lot of files you can use
git add *
or just a single file
git add <filename-which-changed>
git commit -m "this is an explanatory message"
You will need to provide your github user/pass for this next step, or have ssh keys setup in your github account.
git push
Back in your browser, navigate to https://github.com/YOUR-USERNAME/lightning-dev-site and select "Pull Request".
Github will check your fork against the original and highlight the changed files.
Proceed to add the PR and include a useful message.
See: https://help.github.com/articles/creating-a-pull-request-from-a-fork/