Guide me abt how to practise sums of costing nd accounts?

guys plzz tell me how to practice sums in costing and accounts.. wenevr i sit fr dng sums nd attend the first sum i gt bored nd leave the sums nd start reading the answers of all sums widout practising it... nd if by chance i do more than 3 sums n if i go wrong somwhr my intrest goes frm practicing sums... i use padhukas book for costing nd for accounts i go to mp vijay kumar classes nd follow his notes oly... plzzz suggest me som tips for practising sums... nd plzz tel me wat shall i do wen i go wrong in any step..