[Works on Mobile] Click the attunement structure (The left structure) to begin. It will show you every Meeb type. Click the one you want to attune. Hovering over a type of Meeb shows you how many Meebs you already have of that type. After choosing a Meeb to attune, you will need to choose the element to attune it to. Each mirror island represents an element. Below, you can see which Meeb you have selected with an arrow pointing to the element you are attuning it to. Hovering over the islands will show you the element they represent. Once you have all the Meebs you need, click on the synthesizer (The right structure). It will show you all the attuned Meeb types. Choose three or four of them, then click confirm. Each one represents an element for the monster you going to synthesize. But watch out! The synthesizer is unstable, and it won't work 100% of the time. If successful, an egg will appear in the synthesizer and all the Meebs involved will get unattuned. If it's unsuccessful, you will not synthesize the monster and only one of the Meebs involved will get unattuned. You cannot attune a Meeb to it's own element. For attunement and synthesizing, the timer needs to count up to 100 for it to be done. All monsters and Meebs take the same time.
Credits to @mysinginmonsters scratch.mit.edu/users/mysinginmonsters
