Sketch Synth 2 to start with is a simple XY pad synth; for a producer though it solves an important problem: how do I go from having a bunch of loops that sound good to creating an arrangement where I can experiment with how different instruments play against each other? The app ships with several preset instruments so you can get going straight away and understand how the sequenced XY pads control the arrangement of the sounds; but the most important aspect of the app is to be able to create your own pad based instrument with your own loops.
You can use audio copy and paste to copy in loops from other apps, but the app was designed with it in mind to take loops from a DAW on your main computer via iTunes file sharing not another iPad app. Once youve loaded the samples into the app in Blueprint mode then you can configure the background and assign loops and FX to every pad on screen and start arranging.
All XY pads are rapidly reconfigurable and you can quick fade sounds in and out to understand when and where you should be bringing your loops in and how fast they should come in. Once you have worked out how everything works together then you can record in Sketch Synth, but really you should be going back to your DAW and building the stems properly.