Just got a GOTG Pro on Friday - my first pinball machine! I added Cleland's sound mod, and it's fantastic. Cleland, thank you sir! If you have a gofundme or anything where I can support your efforts, please let me know!

For complete newbies like myself, don't be afraid to do this - it's REALLY EASY! Here are the step-by-step instructions I used (gleaned by reading the posts in this thread) - the computer I use is a Macbook Pro:

1. Go to post #364 in this thread and download the appropriate file for your GOTG machine (Pro or LE/Premium) on to your computer.
2. Go to https://etcher.io/ and download Etcher on to your computer.
3. Pick up a blank SD flash card - I got this Sandisk 16 GB card from Amazon, per some of the other posters in this thread, and it worked like a charm (it was about $9). https://www.amazon.com/gp/product/B0143RTB1E/ref=oh_aui_detailpage_o01_s00
4. Insert the flash card into your computer (on my Macbook Pro, the slot is on the right side)
5. Open the Etcher app to use it to move the Cleland file on to the SD card - just click on Select Image, then find the GOTG Cleland image (probably in your Downloads folder), then Click Select Drive, select the Sandisk card, and then click Flash. When it's all done, eject the new SD card from your computer.
6. Open the back box of your machine and lift out the back glass.
7. Turn off the power of your machine. Make sure all of the lights on the CPU (which is right in the middle) go off.
8. At the upper right of the CPU is where the stock flash card is housed (the top of the flash card should be peeking up over the top edge of the CPU). Push down gently on the stock flash card, and it should pop up. Remove the stock flash card and insert the new SD card until it clicks in (make sure that the metal contacts are pointing toward the back, and that the end that the contacts are on goes in first - the label of the SD card will face you and likely be upside down).
7. Power your machine back up - it takes a moment for the machine to complete the installation. Put the back glass back in, lock the back box, and you should be good to go!
8. Keep the stock flash card and update it when new code is released so you have a back-up.
9. Tell Cleland how awesome he is!

ADDENDUM

Step 1 should now refer to post #506 for the latest version of code (1.06).

ADDENDUM 2

Step 1 should now refer to post #772 for the latest version of code (updated soundtrack!)