Clone a copy of your forked IGB repository onto your computer. You will make changes to your local clone, commit them to your local repository, and then ultimately push your changes to your fork hosted on Bitbucket.
To clone your fork, use git clone with the address of your remote fork. (Copy the address from your fork's Overview page at Bitbucket.)
For example, if your fork is named my-igb-fork, you would execute
git clone <ADDRESS>
where ADDRESS is the address of your fork on Bitbucket. To get the address of your fork, look at the top the top right of your fork's home page on Bitbucket.
Make a branch
Before you start making changes to your local clone, you should first create a new branch for the changes you intend to make. This will allow you to issue focused, low risk pull requests that can be easily merged with other branches of development.