there's two basic ways to do it, you hook up each keyboard one at a time to your line inputs as needed. restricts you to using only one at a time.
since most electric keyboardist i've had experience w/ use two or more keyboards in the same song, and you already have a mixer, here's what i'd do.
the last to channels on your mixer are stereo channels, which means one fader adjusts the l/r of your keyboard outs simultaneously. just plug the L/R outs of your keybords to the L/R inputs of the mixer.
-if- you have more than 2 keyboards, then the addition ones will have to be plugged in to 'pairs' of channels. so keyboard for 3 (and more), starting w/ channels 1 and 2, hookup your left out, to line in 1, right out to line in 2. Now very important. you have to pan ch1 all the way to the left, and ch2 all the way to the right. keep doing this for any additional keyboards.
Now plug you line outs from the mixer, to the line inputs on the front of your interface. plug in your interface via usb. your all hooked up.
One thing to remember is that your interface is capable two tracks at a time. so even tho you have multiple keys hooked up, and are playing them at the same time, the collective performance, will be recorded to the same two tracks.
if you want to 'layer' keyboards, or have each keyboards have it's own track/tracks, you will have stick to using one keyboard at a time and just keep making new tracks, and overdubbing them one key at a time.
if you choose this approach, i recommend using MONO tracks, recording the left channel only. think about it, if you have eight parts all hard panned left and right, you'd basically have this big mess, and not much happening in the middle. Panning mono instruments in the stereo field allows each part to have it's own space and be heard.
OK, there is a very very important concept in audio called 'gain staging', this basically is the technique for getting a clean, quiet, full sound. here's what you do:
turn the volume on your keyboards about 3/4 of the way up. Place the faders on your mixer at '0', or 'unity gain'. often it's a shaded area on the mixer. now pick your sound, and play as hard as you normally would. while looking at the output meters on your mixer, starting completely counter-clockwise position, turn the "trim" knob (usually at the top) on your mixer gradually up until the meters are reading around half way up, or a bit more. you don't want to see yellow.
Rinse was repeat for each keyboard. Next bring the master fader on your mixer up to unity.
w/ the 'trim' or gain knobs on your interface, all the way counterclockwise (off), gradually turn them up while watching either the meter ring on your interface, or the meters on the track in mixcraft. when you see them around halfway up, your good. that's it, that's gain staging.
You may be wondering well "why am i leaving so much room on the meter", the reason is because digital distortion is a nasty undesirable thing. you want to make sure that no matter how hard you hit your keys you never even come close to the top. Also, when it comes time to mix your tracks are going to inherently get 'louder'. if you max them out on the way in, you'll have no room left for processing. hence the term headroom.
if for instance you add 6db of highs, your signal is actually getting 6db louder. headroom is key.
any book geared toward home recording, or basic digital audio recording, should be fine, i don't know of any off hand, that are especially good.
Good luck! happy recording!
