When should you use smelt?

If you mean smelt as in the past tense and past participle of smell, then:

use the past - smelt when talking about something that happened in the past and is now finished eg The dog smelt the smoke and began barking.

use the past participle - smelt when:

using present perfect - I have smelt that smell before somewhere.

using past perfect - The lions had smelt the smoke and were now agitated.