Advice from above is to only trim if necessary.
Routine trimming of all sheep is very much frowned upon as trimming often will make the hoof grow faster so needing trimming more often, and if overzealous can lead to inflammatory tissue forming and 'strawberries' or toe granulomas.
Trimming of lame sheep is only recommended to diagnose whether it is scald, white line problems or footrot.
If an infectious problem, it is best to give antibiotic - usually spray for scald, spray + injectable for footrot, and anti-inflammatory painkiller, and then GENTLY trim back the loose dead horn after a couple of days, by which point it should be less painful and the disease on the way to healing. This seems to help them recover more quickly and with fewer recurrences.
Hope that helps