Open Google Classroom
Press the classwork TAB
Open the assignment you need to complete
Open the attached document
Press the Squarrow (square with an arrow) to open in docs
Press the pen tool to edit the document
Complete your work
Press the back arrow to close
Then press x to close
Go back to Google Classroom and press the Hand in button
If you need to edit the document again or add anything more to the assignment press un-submit. edit your doc or add/delete as appropriate and hand in again once finished.