Have a Lot of Resources Ready

 There are different reasons why you will not be able to do certain break up spells. For example, some spells are supposed to provide immediate results to break them up. The moment that you try them though, the effects do not last.

https://protectingtheworker.com/articles/details_you_should_know_about_the_spell_to_break_up_a_couple.html

You need to familiarize yourself with the spell that you want to cast. The more that you know what to do; when to cast the spell, and where you should do it, the more that it will be guaranteed that the spell will work for you.