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First, to understand how blood flows through the heart, you need to have a basic understanding of the structure of the heart. The heart has four chambers: two that are on top called the left atrium and right atrium (or atria), and two that are on bottom called the left ventricle and right ventricle. These chambers are each interconnected by unique structures, which will be bolded for emphasis throughout this explanation. For each step, I will indicate whether the blood is oxygenated (contains oxygen, coming from the heart) in red, or deoxygenated (does not contain oxygen, going towards the heart) in blue. 

How blood flows through the heart:

• Deoxygenated blood from the rest of the body enters the heart through the superior vena cava and inferior vena cava. The first chamber of the heart that the blood enters is the right atrium.
• Once blood has entered the right atrium, the tricuspid valve at the bottom of the atrium opens, letting the blood flow into the right ventricle.
• When the right ventricle fills up with blood, it contracts/squeezes to close the tricuspid valve. At the same time that the tricuspid valve closes, the pulmonary valve opens.
• The pulmonary valve opens to send blood into the pulmonary artery. Remember that pulmonary means lungs, so the pulmonary artery is sending this blood to the lungs to become oxygenated and release carbon dioxide.
• Once oxygen exchange within the lungs has occurred, this blood is now oxygenated and needs to reenter the heart. It does this by traveling through the pulmonary veins, which empty blood into the left atrium of the heart. 
• Once blood is in the left atrium, the bicuspid valve at the bottom of the atrium opens, and the blood flows through to enter the left ventricle. 
• When the left ventricle fills up, it contracts/squeezes to close the bicuspid valve and open the aortic valve at the same time (similar to what happened on the right side of the heart, just different structures!).
• Blood is sent through the aorta to the rest of the body, where it disperses oxygen to various body parts and systems. This blood becomes deoxygenated after flowing through the body, and this is when it returns to the heart through the superior and inferior vena cava. Then the cycle repeats!

Some rules of thumb:

1. Generally, blood on the right side of the heart is deoxygenated and blood on the left side of the heart is oxygenated.
2. The tricuspid and bicuspid (sometimes called mitral) valves are similar structures, but they are definitely not the same. The tricuspid has three “cusps” that close the valve off, while the bicuspid only has two. Along with this, the tricuspid is on the right side, and the bicuspid is on the left. These two can be easy to mix up– just remember: you ride a tricycle (tricuspid) before a bicycle (bicuspid)!
3. Another mnemonic to memorize the flow of blood through the heart is “Try Pulling My Aorta":

Try = Tricuspid

Pulling = Pulmonary 

My = Mitral (bicuspid; you can remember that "my" rhymes with "bi" if you don't like the term "mitral")

Aorta= Aortic 

This can help you remember the order of valves through which blood flows!

4. If you are having trouble remembering these steps, you may need to spend more time familiarizing yourself with the structures of the heart. Be sure to study the bolded words first!
5. Once you can explain how blood flows through the heart to somebody else without having to reference notes, you know you fully understand the concept.