objective: byte summation of whole message without the header followed by two's complement of the sum.

Header: 0xAA
Message Type: 0x01
Message Length : 0x00
Checksum: 0xFF

Question:
1) how to do a checksum?
2) Is it to say adding of 0x01 and 0x00 will give me 0xFF or ???
3) How to send the bytes over?