Allow your cakes to cool on a cooling rack.
Now, place your 8×3 cake on a cake board (cake board should be the same size as your cake). Cut it in half and fill it with strawberry frosting. Then, cover the entire cake in frosting. This will be your bottom tier. Keep it in the refrigerator while you work on your other tier.
Cut the three 6×3 cakes in half and fill them with frosting. Then, stack them on top of each other. Cover this entire stack of cakes in frosting. This will be your top tier.
Roll out marshmallow fondant (I colored mine bluish grey) and cover your top and bottom tier in fondant.
Insert dowels or bubble tea straws into each tier for support.
Then, stack the two tiers on top of each other.
Bring out your homemade heart sprinkles. Brush some corn syrup or royal icing behind the sprinkles and start sticking them onto your cake.