Hi. The whole process is actually very simple. The secret lies in the mixer mode in Sibelius. Here's what you do:
1. Open Sibelius.
2. Open the GPO Studio
3. Click on the Play menu at the top of Sibelius, then move down and click on devices, then turn on the GPO Studio Devices (you should have 8)
4. Set up your score.
5. Load the corresponding samples into however many instances of the Kontakt player you require.
6. Go back to sibelius and type Ctl+Alt+M (opens the mixer console)
7. Now, once you have the mixer open, click on the first instrument name (you should see it under the slider, right beside "Click"). Once you click on the name, you should see to the right a series of boxes. The first should be the instrument that you chose, second should say "device", and third should say "sound". Here's where you do all of the setups.
I'll give you an example.
Say you are writing a piano piece. You've already set up your score in Sibelius and loaded the correct sample into the first slot on the first instance of the Kontakt Player in the GPO Studio. Now, I go into the mixer and click the "Piano" track. To the right, under devices, I'm gonna select "GPO Studio 1", (this sets up the GPO Studio to be the output device) then, I'm going to move down to the far bottom right of the mixer console to where I see a box with the word "Program" beside it. Since I loaded the sample into the first slot (or Channel 1) of the GPO Studio's Kontakt player instance 1, I need to set the program to channel 1. Now, if you go up and click the "test" button, you should hear the GPO piano banging away.
I hope this helps. Please let me know if I can be of any futher assistance.