I have many books,dvds,cards,etc on football.I rate a player taking an account of the following
1.His statistics like number of appearances for the club,national team,goals scored if he is a forward or an attacker,for a defender or a midfielder I rate based on his reputation and of course his performance in the team.
2.The club/national team for which he has played for[this is very important and you will be in need of this knowledge often].If he has played for a reputed club and the club has won trophies in that era then his rating is likely to be high.
3.Also the league in which he has played matters for example an average player from an asian/african/any minor league will be rated in 50s or high 40s,depending on his contribution to the team.Whereas an average player from Italian,English,German league would be automatically awarded a rating of high 60s or low 70s.[This again depends on the club for which he has played].
4.Also you have to take into account how good the league system was at that point.For example the Dutch league reached its peak in the late 60s till then their league was in wilderness.Therefore rating would be definitely higher from that point.The dutch league lost its flair in the mid 90s,therefore there should a dip in the rating from then on.As you are from Belgium,you must be knowing the best years of Belgian football were the 60s,70s and 80s.Especially the mid 70s and early 80s.The Belgium national team was very strong,the clubs were highly successful in Europe hence the rating would be automatically high especially for the clubs that performed well in those years.Similarly English football took a severe blow in the mid 80s and early 90s and success was limited.
5.As you can see it is a complicated system that I use.But I feel I am able to arrive with the best results if I follow this.You need a bit of practice but soon you will find it effective.
PS:If you have any doubts in rating teams send me the list and I will give you an idea of how to rate them...GOOD LUCK!!