
Create a region or region group ( OPT+CMD+G) the EXACT length of the original highlighted loop (before duplicating). Test the smoothness of the transition and adjust the crossfade shape and length as needed.
Duplicate the file (use CMD+D to ensure sample accuracy) and cross-fade the two together without shifting their positions in any way. Copy and paste the highlighted area onto a new area on the track.
Following the natural pattern of a waveform is especially important if the source material already has a noticeable rhythm to it. Pay attention to the In and Out waveform sizes and try to match them.
Using waveforms as your guide, highlight a section that is close to your ideal loop. Consolidate any effects you want on the source material, and bypass any active effects you have on the track you will be working with. Set your session to loop playback, and have 2 side-by-side Audio Tracks available for use.