I'm not sure what git client you're using. Assume you're on Windows and you have Git Bash, try these steps:

• On page https://github.com/steemit/steem , clone the repository to your account by clicking "fork"
• open Git Bash
• git config --global user.name="your_github_user_name"
• git config --global user.email="[email protected]"
• git clone https://github.com/your_github_user_name/steem.git "your_new_path_to_the_project"
• cd "your_new_path_to_the_project"
• git checkout v0.14.2
• git branch "your_new_branch_name"
• git checkout "your_new_branch_name"
• git submodule update --init --recursive
• Copy your modified source code (tree) to "your_new_path_to_the_project"
• git diff
  (use above step to check whether the new code is correct)
• git commit -a -m "your_commit_comment"
• git push origin "your_new_branch_name"

Then you'll see the new branch on github, you can create a pull request from your branch so the changes can be merged to upstream repository.

If you have modified code in libraries/fc, it would be a bit more complicated, you may need to

• On page https://github.com/steemit/fc , clone the repository to your account by clicking "fork"
• Edit the file ".gitmodules" in "your_new_path_to_the_project", replace the URL in the file to your FC library
• Remove the "fc" directory from "libraries" folder
• cd "your_new_path_to_the_project"
• git submodule --init --recursive
• cd libraries/fc
• git branch "your_new_fc_branch_name"
• git checkout "your_new_fc_branch_name"
• Copy the modified FC code here
• git commit -a -m "your_commit_comment_to_fc_lib"
• git push origin "your_new_fc_branch_name"
• cd "your_new_path_to_the_project"
• git commit -a -m "your_commit_comment_to_the_main_project"
• git push origin "your_new_branch_name"

With these changes your repository would be able to compile independently, but it can't be merged directly to upstream repository (due to the change on .gitmodule file).

//Update: to upload binaries, follow the instructions on https://github.com/your_github_user_name/steem/releases