There is a ton of useless information in this spec that's just there to confuse you.

For every command, bytes 0-4 are provided for you. The 5th byte and on are the parameters for a command. The las two bytes are the checksum. The checksum is simple addition.

So for power status:
07
01
30
02
00
If we add those together we get a sum of 3A. The unit wants to see a 2 byte sum. Because our sum is only one byte we need to add some leading zeros making the sum 003A. Next, the unit wants the bytes reversed, making the checksum 3A 00.
The complete string would be:
07
01
30
02
00
3A
00

Now let's try Select Analog Input 4:
08
01
30
04
00
04 (Decimal 4 converted to HEX)
41
00

I don't use RTI so your on your own for proper formatting.