Step 1: Turn each price into a percentage
Betstrike shows odds in decimal format (for example, 1.83). With decimal odds, the chance an outcome implies is simply:
Implied chance = 1 / odds
Work this out for every outcome in the market, then add them together. The total is called the overround - the size of the book.
Step 2: Worked example (a 1x2 match)
Take a match with these odds - Canada 1.83, Draw 3.50, Bosnia & Herzegovina 4.50:
Canada (1): 1 / 1.83 = 0.5464, or 54.64%
Draw (X): 1 / 3.50 = 0.2857, or 28.57%
Bosnia & Herzegovina (2): 1 / 4.50 = 0.2222, or 22.22%
Overround: 0.5464 + 0.2857 + 0.2222 = 1.0544 (the book totals 105.44%)
The 5.44% above 100% is the extra built into the market. A market with no margin would add up to exactly 100%.