Adding of submodule and subproject in git is very simple and straight forward. Let us follow step by step command on your git terminal and you are done.
Step #1: to checkout the existing project
git checkout BRANCH_NAME
Step #2: to add submodule in git project
git submodule add GIT_URL PROJECT_NAME
Step #3: to update the files and folder recursively
git submodule update --init --recursive
Step #4: to check the status
Step #5: to commit the code into local repo
git commit -m "External Path Created"
Step #6: to Check the status
Step #7: To push the staged/committed project to remote git branch
Your suggestions are welcome to encourage me to write and improve this post.