I found this to be glitchy if I had any normal Goat Sim mods on (mods that existed in previous game such as double jump). What I found was that I had no inventory if any of these were turned on and even with taking them off I had to restart to get my inventory back. Would recommend doing this achievement on it's own if you find the same.
From the beginning head up the hill and down into the fields below and then into Gaotshire. To the right side of the town is the tavern, inside is a quest giver who will ask you to find Miranda the Mermaid. Head out of the tavern and follow the river to the waterfall to the right. Jump down and follow the left hand side until you find yourself on Glarblargle Beach.
From here carry there will be Miranda the Mermaid ahead and to the left. Lick her
, then open your inventory up
, add her to it
and finally close it
Then all you have to do is head back to the tavern with Miranda in your inventory. Some guides say to remove her from your inventory but I didn't need to.