How do I connect a bass guitar to my iPad to use with Garage Band
Hope this will help?
http://www.musicradar.com/news/guitars/6-easy-ways-to-connect-your-guitar-to-your-iphone-ipad-or-ipod-touch-516544
Or even more specific:
https://gigaom.com/2013/02/09/four-ways-to-connect-your-instrument-to-garageband/
Or you meant something different?