The bobbin in a sewing machine contains thread to create the bottom part of a seam. The bobbin can be plastic or metal. The bobbin fits into a bobbin case and that case is inserted into the machine from a location where the needle thread can loop with the bobbin thread.
Fill the bobbin with thread. Pull out about 6 inches of thread from the bobbin. To correctly place the bobbin into the bobbin case, hold the bobbin case in your left hand. Pick up the bobbin with your right hand so that the thread wrapped clockwise when you drop the bobbin into its case.
The loose thread from the bobbin needs to be threaded through the bobbin case. To do that, with your left hand you can pull the tab on the back of the case, which holds the bobbin. Grab the loose thread with your right hand and pull it into the slit on the bobbin. Tug the thread down, and under the tension spring. When tugged properly, the thread will be between the "fingers" of the tension spring.
To install the bobbin into the machine, pull the tab on the back of the bobbin and while grasping the tab, slide the bobbin into the sewing machine's bobbin location.
With the bobbin pushed all the way in and the bobbin arm matching the slot in the bobbin shuttle, release the tab. The bobbin is now installed into the machine.
With the needle already threaded, use your left hand to grab the thread from the needle. With your right hand, slowly turn the machine's hand wheel toward you. As you turn the wheel, the bobbin thread will loop up and become visible. Stop turning the wheel when you see the loop from the bobbin thread. Grab the loop with your right hand and gently pull the thread loose so you can lay the bobbin thread with the needle thread.