1) Select the two tones that you want to layer (please note that you can only layer tones that are in different categories from one another - not two tones from the same category).
2) For example, to layer piano and strings, first you would select the piano sound that you want to use by holding the "piano" button, followed by the corresponding piano key that marks which piano sound you want.
3) Next, you would hold down the "other" button, and press the lowest note on the piano to select Strings.
4) Now that both tones have been selected in each category, you would press the "piano" button and the "other" button at the same time to layer those 2 previously selected tones.