"Be simple" is easy enough. Just focus on how you will do whatever you are doing without letting unhelpful thoughts coming inside your mind to distract you from what you want to do. For example if we are about to do peng qi guan ding fa and we want to bring qi into our body we are taught to just think inside our body and not bother about how qi will enter our body. Any attempt to visualize how energy spirals or rays of light entering say our 'bai hui', 'yong quan' etc will only distract us and make us lose the relaxed state of mind so essential to effective execution of peng qi guan ding fa.
If you are thinking: Isn't it better if we can visualise how qi will come into our body through the apertures and be distributed throughout the body via the meridians? The answer lies in our mind being the commander of qi. In fact, an experienced practitioner who has abundant qi will sense qi move the moment his mind move. So just relax and "Be Simple!". Qi gong is NOT difficult ;) Just focus inside your body and qi will follow mind into body.
What about merging with the void? Some said that the void is some far away place where there is abundant hunyuan qi. While it is true that void can exist in that far away place and have abundant hunyuan qi, but believe me; for a qigong practitioner to think of this 'elusive far away place' is not simple. Would it not be easier to focus on the void within our body? Despite our body's appearance of being solid the reality when it comes right down to the atoms and sub-atomic entities that made up our body we are in fact essentially 99.999% (not sure how many 9's) void! With this realization perhaps we may understand why the eight verses starts with:
"Ding tian li di..." (Heads touches the sky, feet planted deeply into ground). Remembering that we must stay simple: the way to feel ding tian li di, would be to think of our head rising into the blue sky and then when we look down from the sky, our feet seem so far and deeply planted on the tiny earth below. Here again, if you have abundant qi, you will feel your entire spine stretching as you sense your head moving up into the sky. ;)
The second verse: "Xing song yi chong" means we simply relax and feel our body expand in all directions, front back, left and right. Our body now fills the vast expanse of heaven and earth... And now isn't it easy to experience the void right within our body when it is stretched in all directions - the four directions plus up and down? In this state just focusing our mind within our body will merge our mind with the void within. Thinking of that far away place if one cannot stay or feel qi within can easily lead to our mind straying away from our body leading to what we termed: "Separation of mind and body." Something qigong practitioners want to avoid.