@abit: I'm not sure

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="your_github_user_name@users.noreply.github.com"
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