If ya mean copying a solo off a CD or whatever... I'll usually break the solo up into sections and work on each part. If it's really difficult I'll set my CD player to repeat the solo endlessly.. do a basic copy of the background music in midi or time stetch it to slow it down. (course ya used to be able to use 16rpm speed but nooo...somebody had to invent the CD)
If I'm really stuck I'll search the web for any tabs available.