GMT's Minigolf is a very relaxing gamemode. You can sit back, and listen to the ocean waves lap up on the shore, and hear birds chirping while you and everyone else putts. So, don't fret.
Minigolf, rather unsurprisingly, works like miniature golf works in real life. Try to get your ball in the hole with the least amount of strokes possible.
Being one under par will give you a birdie. Two under par is an eagle. Three under par is an albatross.
If you get a hole in one, you win a free game! However, the game's free already. Oh well.