If you are the one who likes this extra level of control in your diet and supplement, there is no reason why you couldn’t do your own preliminary training. Well, scratch it, there are several reasons: it can be much more expensive, and depending on how you do it, you can skip a few key ingredients. And the taste will probably be terrible … Yes, this is a fairly high order for the average goalkeeper.
So, before you put on a lab coat and go into PubMed, make sure that you have a solid foundation for training and nutrition. Do you train 3-5 times a week consistently and in good shape? Do you eat enough calories and protein and have a solid preliminary workout before intensive workouts? Check, check, check. Well, then maybe you are no longer the “average” athlete.
However, before you start exploring the dark web for the discounted palette of pure African yohimbine, keep in mind that starting is simple – this is almost always the best option. If you're going to make your own, keep it in key ingredients that share the best pre-workout: caffeine (if that's your thing), beta-alanine, and possibly nitrous oxide boosters for endurance and pumps. If you want to go down to the rabbit hole of nootropes (i.e. supplements to enhance the brain) or other highly intelligent ingredients, go for it, but don't expect magic.
Remember that this is not about cheating the process. This is only a reasonable improvement in what you can get from it!