For the proficiency you need to have a certain amount of experience in football betting in general and lines in particular and you need to practice this to fully comprehend.
example:
Let's say in a very close match, you couldn't decide on what to bet. The odds are as follows:
A to win: 2.9
Draw: 3.5
B to win: 2.8
If you bet 10€ on A to win, your P/L would be 19€ profit or 10€ loss. Then to cover this, you need to bet on B to win with an amount that can get more than 10€ profit, meaning you need to bet with something between 6 - 18 Euros. If you bet with 12€ then you would get 21.6€ in profit and even if the other bet loses (A to win) you could at least get 11.6€ more but only if B wins. Or vice versa in case A wins and B loses then you would get 7€ profit.