This is my preferred way of installing files but you use any method that you like. I use Windows Explorer.

1) Make a folder called "Temp" or "Crud" or anything you like in the root directory of Windows Explorer.

2) Unzip all the files and folders from the downloaded "su_31xxx.zip" file to the folder you just made (for example "Temp").

3) Select the folder called "Sukhoi_SU_31" and hit "ctrl-c" to copy the entire folder and its contents.

4) Go to the "Aircraft" folder of your flight simulator and paste (ctrl-v) the copied "Sukhoi_SU_31" folder there.

5) Go back to the "Temp" folder. Select and copy the "SU-31.CAB" gauge file from the "Gauges" folder.

6) Go to your flight simulators "Gauges" folder and paste this new gauge file there. Be sure to read the "READ_ME.txt" and "FS2004_Flyers.txt" files in the "Temp" folder. That is it. If you wish you can delete your "Temp" folder now but I would save the downloaded "su_31xxx.zip" file somewhere just in case of emergencies.

7) Go fly your new aircraft.

Enjoy