How do I convert float to byte[] by using actual logic.

That's complicated in C#, because casting doubles to "byte friendly" values in C# isn't possible in "normal" code.
If you have a look at the Reference Sources however, you can see how they do it - and it's pretty simple:

[System.Security.SecuritySafeCritical]  // auto-generated
public unsafe static byte[] GetBytes(double value)
{
    Contract.Ensures(Contract.Result<byte[]>() != null);
    Contract.Ensures(Contract.Result<byte[]>().Length == 8);

    return GetBytes(*(long*)&value);
}

[System.Security.SecuritySafeCritical]  // auto-generated
public unsafe static byte[] GetBytes(long value)
{
    Contract.Ensures(Contract.Result<byte[]>() != null);
    Contract.Ensures(Contract.Result<byte[]>().Length == 8);

    byte[] bytes = new byte[8];
    fixed(byte* b = bytes)
        *((long*)b) = value;
    return bytes;
}

That's the most efficient way to do it (ignore the Contract bits)
But unsafe code may not be what your homework asks for either...