First off, a Full house (also called a boat) is when you have three of a kind of one type of card and a pair of a different card at the same time. For instance having a pock hand of A-Q with the board being 7-A-Q-9-A is a full house as you have three Aces and two Queens.
It is important to note for this achievement that when you split a pot (which occurs when you and another player use the same hand) you are still considered to have "won" the showdown. This means that you do not have to beat a player with a full house by having a better full house, as you can both have the same full house and both technically win the showdown.
This brings about two different scenarios (please note it does not have to occur exactly like these, but in a similar fashion) that will net you the achievement.
First Scenario
You: J-9
Opponent: K-7
Board: 3-9-7-3-3
In this case both you and your opponent use the three of a kind 3's on the board, and you both make an additional pair with the board, but since you have a pair of 9's and your opponent has a pair of 7's, you win the showdown and earn the achievement.
Second Scenario
You: A-8
Opponent: Q-8
Board: J-K-8-J-8
In this case the board shows both a pair of Jacks and a pair of 8's, but then both you and your opponent have an 8 in your hand giving you each three of a kind 8's as well as a pair of Jacks, so you both have the same Full House: 8's full over Jacks. You would split the pot in this case, but both of you would "win" the showdown in the game's eyes and both of you would earn the achievement.
This achievement will come in time and with some luck, but do not worry too much about it because these situations happen much more often then you would expect. Just keep on playing and this will happen eventually.
I hope this was helpful, and I know it was probably a little convoluted so please message me or leave a comment if you have any questions.