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The following is based on just getting gyro attitude information, although it includes some information about using MSP in general. The example code referenced can be found here. The most important part of this that I couldn't find anywhere else (a friend had figured it out and let me in on the secret) is the Data section below.

Firstly, let's look at how MSP works. I found this link (alternative link here) very useful in understanding this, but I'll summarize here. There are three types of message that can be sent.

1. Command -- a message sent to the flight controller which has some information to be sent.
2. Request -- a message sent to the flight controller asking for some information to be returned.
3. Response -- a message sent by the flight controller with information responding to a request.

MSP messages have a specific structure. They have a header, size, type, data and checksum, in that order.

I found this diagram from here very helpful:

The header is three bytes and contains the message start characters "$M" and a character showing which direction the message is going. "<" denotes going to the flight controller (command and request), ">" denotes coming from the flight controller (response).

The fourth byte is the length (in bytes) of the data section. For example, if the data section had three INT 16 variables then the size byte would be 6.

The type byte specifies what information is being sent in the message. You can find a list of types here. An example of this would be MSP_ATTITUDE which has type number of 108.

The data is where all the information is sent. Request messages have no data in them. Commands and responses do, because they contain information. The types of data returned can again be found here.

The difficult part of the data section is that the bytes are reversed in order, and this is extremely poorly documented. So, for example, if I get the following two bytes in this order: