Sequential download is a Pro feature in  Torrent Pro app, which is provided for free. You can enable it in the Settings page, and then you will find the sequential download option in two places:
1. You will find the sequential download option while placing a download. In the torrent add dialog, you can find the option and enable it.
2. If you have already added the torrent, then you can click on the torrent file, which will take you to the details page. In the details page, under the 'INFO' tab, you can find the 'Sequential Download' option, where you can tick mark it to enable sequential download.
Once you enable the sequential download, Torrent will try to download the pieces in sequential order (1,2,3,...n) for that particular download only. So, whichever torrent you want to download in sequential manner, you can enable it.
This feature is very recommended while you stream your torrents while downloading.