This achievement is very glitchy. Do not panic if it doesn't unlock the first time you meet the requirements, it will unlock eventually. Best to do a quick match on easy (though it can be done in any mode, offline or online). A conversion is a kick for goal taken after a try is scored. Try to score under the posts, as the kick is taken in relation to where you put the ball down. In the event that you scored out wide, allow for the wind, take your time and do your best to get the bar to stop in the green (inside the posts on the meter).