How to do it...

1. To construct a quaternion, the best constructor to use is as follows:

FQuat( FVector Axis, float AngleRad ); 

Quaternions have quaternion addition, quaternion subtraction, multiplication by a scalar, and division by a scalar defined for them, among other functions. They are extremely useful to rotate things at arbitrary angles, and point objects at one another.

2. For example, if you wanted to use an FQuat inside of the RotateActorComponent.cpp file, it would look similar to this:

void URotateActorComponent::TickComponent(float DeltaTime, ELevelTick TickType, FActorComponentTickFunction* ThisTickFunction)
{
    Super::TickComponent(DeltaTime, TickType, ThisTickFunction);

    // 11-04 - Rotation using FQuat
    FQuat quat = FQuat(FVector(0, 1, 0), GetWorld()->TimeSeconds * PI / 4.f);
    GetOwner()->SetActorRotation(quat);

}

Upon compiling your code and returning to the game, you should notice the cube moving at a constant rate: