STEP 1: Make sure you don't have the assignments you want to sync filtered out in your gradebook.
STEP 2: Click "Sync Grade with Classroom"
STEP 3: Select the assignments you want to sync from the popup
View grades in Google Classroom
STEP 1: Select the class and "Grades" tab.
STEP 2: When you are ready to send a Grade to the student, click on the 3 dot icon next to the grade and select "Return."