Getting Started

Installation

You can install the addon one of two ways.
1) Extract the onion_skin_tools folder from the .zip file and move or copy the folder directly to your
/2.8x/scripts/addons directory.  
2) From the User Preferences window, navigate to the Addons tab, and select the Animation filter.
3) Click the checkbox next to “Animation: Onion Skin Tools” to enable the addon.
4) Click “Save User Settings.”

Features & Settings

This is the addon panel. After enabling the addon, you will find this panel in the 3D View > Right sidebar > OST tab.  Familiarize yourself with the following settings.  
A quick note: In this documentation, we’ll be verbing the phrase “onion skin” for the sake of convenience. Hope you’re okay with that.

Going Further

Usage

Once you’ve familiarized yourself with the settings, using the addon should be fairly straightforward. Briefly, the steps are:
1) Add objects to the list of objects to onion skin.
2) Adjust frame range settings.
3) Click Run.
4) After animating, click Remove to remove onion skinning from the viewport.

Adding Objects

To add objects to the list for onion skinning:

1. Select one or more mesh objects.
2. Click the '+'  button to add the selected objects to the list.
3. If the selected object is an Armature, the armature will not be added to the list. Instead, any mesh objects parented or deformed by the Armature will be added to the list. This way, you can quickly add a whole rigged character to the list for onion skinning.
4. To add objects from a linked group (say, a linked character rig), select the group Empty and click the '+' button to add the linked objects to the list.

Removing Objects

Click the '-' button to remove the selected list item.

Adjusting Settings

Enable Current Frame Only to run onion skinning on only the current frame. Enabling this will remove several items from the user interface (specifically, the Include Current Frame, Direction, range Backward / Forward settings).  

Keep Current Frame Only disabled to run onion skinning on a range of frames.

Enable Include Current Frame to include the current frame in the onion skinning.

Set the frame range direction.
Backward will only include frames earlier than the current frame. Forward will only include frames later than the current frame. Both will include frames earlier and frames later than the current frame.

Set the frame range. The Backward slider sets the number of frames earlier than the current frame to onion skin.  The Forward slider sets the number of frames later than the current frame to onion skin. The Frame Step slider sets the number of frames to progress between onion skinned frames. A value of 1 will onion skin every frame. A value of 2 will onion skin every other frame, etc.

Now you can run the onion skinning process by clicking Run.

Settings That Update In the Viewport

The following settings will update the onion skinning objects in the viewport. Therefore it is not essential to set these before running the onion skinning operator.

The Earlier Color and Later Color properties determine the color of onion skinning objects earlier and later than the current frame. The Transparency Factor slider is a multiplier for onion skinning transparency.  

Note: After running onion skinning, changing the current frame on the timeline will update the color and transparency of onion skinned frames relative to the new current frame. For example, frames earlier than the current frame will turn blue (or your custom color) and become more transparent as you move forward on the timeline.

Enabling or disabling X-Ray will enable or disable x-ray drawing on the onion skinning objects.

To hide onion skinning objects before the current frame on the timeline, enable Before. As the timeline cursor progresses forward, any onion skinned frames prior to the current frame will disappear from view.
Likewise, to hide onion skinning objects after the current frame on the timeline, enable After. The All checkbox is a convenience - you can use a single checkbox to hide all the onion skinned frames from view, but this is also accomplished by enabling both Before and After.

Removing Onion Skinning

To remove the onion skinning from the viewport, click the Remove button. 

To remove onion skinning only on the current frame (assuming of course the current frame is onion skinned), check Current Frame Only and click Remove.

Tips

You can onion skin a single frame, then progress forward a few frames on the timeline, and onion skin another frame.  Using the Current Frame Only setting, you can onion skin an arbitrary number of frames individually to maintain control over what you would like to see in the viewport while you animate.

Have Fun!