This is my understanding of coaches...
I like to take coaches off doing everything, and put them on only one area, specialisation. Look for regular trainers, rather than youth or first team, because a trainer can complete their specialist training for both senior and junior squads and do it effectively. If not, you may have to look to get both a first team and youth coach for some disciplines.
If a coach has 18 or more in fitness, they will be excellent at strength and aerobic training, though you'd get two, because one person doing both reduces the effectiveness of both.
Look for a goalkeeper trainer with 18+ goalkeeping.
Tactical, tactics 18+
Defending will need defesne of at least 18, but also high tactical. Getting someone with higher defense than 18 (19 or 20) will significantly reduce the need for tactics...but you will need at least 10, with a 20 level defense.
Ball control needs a highly technical coach (18+), but also strong mentality (works like above)
Attacking is the same as defending, just replacing defense with attack...still need good tactics
Shooting needs a technical coach again (18+), but good attacking stats too
Set pieces needs technical of 18+ and also excellent mentality and attack stats.
If you follow that, you should just about get full stars in training. It can take a while sourcing the world for these coaches, because they won't all be world class. There are some basically unknown coaches who have the stats to be good (but they will often only be specialising in one area, which is why they won't be world class, because by default they take all training). Some will require compensation to their clubs and what not though.
One thing I hate, is when I source the most effective coaches in each discipline, and then all the other clubs come seeking your coach
Edit: Oh, and I should mention, at times it is impossible to get all full stars...because sometimes there simply aren't coaches with the right stats...or one coach you have doing one job, who could also get full stars elsewhere, can't do both jobs perfectly...
So choose the most important stats if one coach is perfect at multiple disciplines. Defense and attack are what I like to focus on if I need to