The easiest/fastest way to get aquafaba  is to buy a tin of canned chickpeas (usually 400 g per tin). Just open the tin and the liquid inside is aquafaba. Another way to get aquafaba is by cooking chickpeas yourself. I generally use a pressure cooker or instant pot to cook raw chickpeas (soaked for 8 hours). Then take out the chickpeas (using a sift) and reduce the left liquid to about half by cooking it on high heat. I would recommend using the tin liquid for consistency and it’s easier to succeed in making this sweet meringue buttercream.