No, you don't do it on random. Let's say you are looking at basketball game X vs. Y. Bookmaker A is giving odds X win 2.5 and Y win 1.33 Bookmaker B is giving

No, you don't do it on random. Let's say you are looking at basketball game X vs. Y. Bookmaker A is giving odds X win 2.5 and Y win 1.33 Bookmaker B is giving odds X win 1.55 and Y win 2.10 What you do is bet on X in A and on Y in B. Let's say that you invest $2000 ($1000 in each bookie). If X wins, you loose $1000 on Y, but you win $2500 from X. Total profit = $500. If Y wins, you loose $1000 on X, but you win $2100 on Y with net profit of $100.