Use a good source which shows you the probabilities (percentage-distribution) of results on the basis of good statistics.
It's not easy to find such a good source - an example would be: statarea.com
The alternatives are statto.com and prosoccer.gr
Then you take the current odds of your favourite bookie (that's the one where you have got the most money in the betting account ;-))
Then you have to compare the statistical percentages with the current odds. Decimal odds can be simply converted into percentages by dividing 1 by the odds (e.g. 1 / 1.5 = 0.666 = 67%)
Arsenal Chelsea 27 39 34
Odds on Chelsea at e.g. Bet365 are 2.40
1 / 2.40 = 0.416 --> meaning 42%
Gives you a difference of 42% - 34% = 8%
Then you can stake those 8% according to the Kelly-strategy. Stake according to Kelly amounts to: budget (e.g. £500) multiplied by 1/10th of the valuation (here 8% thus 0.08) divided by the decimal odds minus 1 (here 2.40 - 1 = 1.40)
Thus, at our match 40 divided by 1.40 = a £28.57 stake
The Kelly-strategy has the advantage that the stake is calculated on the basis of probabilities and your currently available budget.