There are Github mirrors of the source repository here.
Fork the project to a public repo using Github web interface, clone the repo to your computer, create a branch for your changes, push these back to Github and submit a pull request.

In case you don't know how to do that, keep reading.

Create a Github account, this will host your public fork of LEDE source, and will be used for all interaction on Github.

Install git in your PC, and make sure that your local git settings have the right name and email

git config --global user.name "my name"
git config --global user.email "my@email.address"

You might also want to set the default text editor to your favourite editor. If you have a Linux system with a GUI, some choices are geany, kwrite, pluma and gedit.
If you are using command line, nano is a good one.

git config --global core.editor "editor-name-here"

Then follow Github's excellent documentation to Fork A Repo and Create a local clone of your fork

After you have set it up as described, write

git checkout -b my-new-branch-name

to create a branch for your PR (“my-new-branch-name” is just an example name, use a more descriptive name in yours).
All commits you do after this command will be grouped in this branch. This allows to have multiple branches, one for each PR.
To switch between branches you already created, use

git checkout my-branch-name

After you made your changes, write

git add -i

and use its interface to add untracked (new) files and update existing ones.

Then write

git commit --signoff

This will open the git default editor and you can write the commit message.
The first line is the commit subject,
then leave an empty line
then you write the commit description.
This command will automatically add the Signed-off-by line with your name and email as set above.
For example, a complete commit message might look like this:

The best code update.

This is the best piece of code I have ever submitted.
Signed-off-by: John Doe <John.Doe@test.com>

To send your local changes over to your Github repo, write

git push --all

You will be asked your github user and password in the process.

After the code has been uploaded to your github repo, you can submit PR using Github web interface, see again Github's documentation about Creating a pull request