I have been following along this tutorial and have made 3 seperate layers during the process. It says (highlight the layer set, in the layers panel, and merge (layer > merge layer set) the layer set into a single layer) I cant seem to get the 3 layers highlighted at the same time. I can only click on one layer then shift to click on another and thats about it. I need these layers merged but it obvious i'm not doing something right here. Any help is appreciated