Clip the spacers onto stock guide rod. Spring rest is a tight fit but it works. After installing the two rests use the spacers to meet your desired spring compression. Face the channels of the spacers upwards towards the outerbarrel or removing the slide will be difficult. So far I've printed them in PLA but I plan to use PETG in the future. I hope this helps! Now us Glock users can keep up to the cycle rate of those hicappa try hards.
Update: I have been told that the clip style spacers have been known to pop off of the guide rod from time to time. In light of this information I have added new spacers which should solve this problem. These require you to remove the clip which holds the spring on then place these complete circle spacers on to the rod. I have not tested these yet so more updates will follow.