Select an element and go to the Animation panel. Click an animation to apply it to the element.
Make sure to select the animation in the Timeline panel, then choose a different animation from the Animation panel.
Place both elements on the same track, then select the last element. Go to the "Transition" page in the Animation panel and choose one of the animations.
Change the length of the transition by dragging one of the sides.
Select an element. Hover over the property you want to animate and click the keyframe icon that appears. Click it again to remove the keyframe.
If an element already has keyframes on the timeline, you can add more by modifying the element while at a different timestamp:
Alternatively, you can add a keyframe by clicking the stopwatch.
Drag a selection box by clicking and holding the left mouse button, then releasing it. Drag the keyframes around to move them.
You can also move keyframes by dragging the line connecting the two.
Select a keyframe from the Timeline, then change any of its settings in the Property panel.
Right-click the keyframe and choose "Delete". Alternatively, you can press Delete on your keyboard while the keyframe is selected.