Open your project in Premiere Pro
Go to the Effects panel
Expand Video Transitions or Audio Transitions
Choose a transition
Drag the transition to the edit point between two clips
Drop it at the cut or onto the start or end of a clip
Adjust the transition duration by dragging its edges in the timeline
Right-click the transition and select Set Transition Duration to change the length
Use the Effect Controls panel to fine-tune transition settings
Apply the default transition with Ctrl+D on Windows or Cmd+D on Mac for video
Apply the default audio transition with Ctrl+Shift+D on Windows or Cmd+Shift+D on Mac