Tap on the onion icon to enable the Onion Skin to see your previous and next drawings. 
Touch and hold the onion icon on the layer to open a small window with onion skin options.
These parameters are accessible via the settings panel with more options.
To access more onion skin options, go to the settings panel and go to the Onion Skin tab. You can see up to 8 drawings (sheets) before and after your current drawing (sheet).
You can choose to display the onion skin as Colors or Images for either previous drawings or next drawings.
When choosing Colors, you can access the color wheel to set the color you want for either previous or next drawings.
When choosing Images, you can see previous and next drawings in their original colors. It is best to use the Images option when you have images without transparency.
Switch the sliders on and off with the buttons next to each slider. The values of the sliders can be changed independently.
The lock allows you to move all the 8 sliders at once. The option Render Position allows you to choose if the onion skin image is displayed over (Front) or under (Back) the current pixels.
Back
Front
This icon located at the top of the layers pile allows you to open the Onion Skin Bar and access all the onion skin options quicker and the Out of Pegs options. 
You can access all the options related to the onion skin from the onion skin bar.
You can turn off and on the onion skin globally in all the layers with this switch. 
The double color circle allows you to choose the colors of your onion skin’s previous and next drawings with the color wheel.
The menu icon allows you to access the onion skin and out of pegs options.
This is a quick access for the onion skin options that are also accessible via the app settings as seen above.
You can also activate the out of pegs option which allows you to move the onion skin in the canvas.
Tap on a peg to enable the onion skin on that sheet (relative to the current sheet). You can enable up to 8 pegs before and after the current sheet.
Touch and hold on a peg and swipe up and down to quickly change the opacity of the onion skin on that sheet. If the lock of the opacity link is enabled, you will be able to change the opacity of all the onion skin pegs before or after the current sheet.
Touch a peg to display the available options. You can disable the peg or change its opacity.
You can use the dynamic display or the static display of the onion skin bar.
The dynamic display will follow the current frame as you navigate in the timeline.
The static display always remain in the center of the onion skin bar and the eight pegs of either side are visible at all times.
In static mode, you can see horizontal bars across the pegs when there is no sheet in the corresponding frame of the layer.
