Backup and truncate LDF with xp_sqlmaint
How can I go about truncating the LDF after the backup?


Don't. A maintenance plan has no business truncating files, LDF nor MDF. There is no point to shrink something only to have it grown back overnite, at (huge) runtime cost. If the file is large, it means it has to be large. Truncating files is just another way of asking for downtime because it gives a false impressions of free disk, where the disk is reclaimed back during normal operations, but potentially can fail (grinding everything to a halt) because the needed space was meanwhile taken by other files. 

So just don't.

On special occasions after the file has grown due to a one-time incident, a manual one-time truncation of LDF is OK. But not as part of regular maintenance, that is absolutely not OK.