Get a Sweet Item (Battle Cafe Reward)
In order to evolve Milcery, you need a sweet. This means going to the Battle Cafe (which you can do daily) because one of its rewards is a Sweet. The one we obtained is a Strawberry Sweet (but any sweet will do).
There are Battle Cafes is multiple places – Motostoke, Hammerlocke, and Wyndon – but the one in Motostoke is the easiest/most convenient early on. There are five sweets you can get from Battle Cafes. See How to Get All Alcremie Forms for a full list.
If you don’t already have a Milcery, you’ll need to catch one. Milcery’s locations are Route 4 and 2 spots in the Wild Area. Route 4 is the easiest place to get Milcery because in the Wild Area it only appears during 1 weather condition while on Route 4 Milcery is always around.
Tip: Move the joystick very delicately so that you slowly walk through the grass. Be careful not to run into the Pokemon in the overworld, especially because some will chase you down.
Give the Sweet to Milcery
You can give Pokemon items even from the PC boxes but to evolve Milcery it needs to be in your party anyway so start by adding Milcery to your party.
Press X to open the menu > Select Other Items > Select your Sweet > Give to a Pokemon > Select Milcery.
Notice that text on the Sweet item says “When a Milcery holds this, it will spin around happily.” This is your hint.
The Sweet will determine the type of decoration that appears on Alcremie‘s head once it’s evolved.
Do a secret dance
In order to do the secret dance that causes your character to spin and strike a pose, spin the left stick quickly. You can even see a video of us doing this Secret Dance in our Pokemon Sword and Shield Tips and Tricks page.
You’ll know you did it correctly when your character starts twirling on their own and then strikes a pose.
Once you follow all these steps Milcery will evolve into Alcremie!
Link Source: Click Here