There are many products with which to mix music together these days, but it seems that Ableton Live is the new industry standard of music performance software. Not only does it facilitate totally dynamic live loop mixing, but it's quite good at arranging songs with automation to make a high quality mix set. In this guide I will go over step-by-step of how to use it for that exact purpose.
My demo will use Live 5, but should apply to Live 6 as well.
Initial set up
If this is your first time using Live or if you've never used it to mix full length songs before, there are a few settings you will want to change in your preferences. Under Sample, you will want to set the Loop/Warp Short Samples to "Warped One Shot" and the Warp Mode to "Complex". This will do the best job handling full length songs overall.
Creating your project
Make a directory on your PC somewhere and put all of your songs in that you would like to use. In live, just create a new project in the File menu. When you go to save your project, make sure to save the live project file into the same directory as your songs. This is your project folder, and it's good to keep it all together so you don't lose anything later from moving songs, etc.
Live uses 2 views: Session view and Arrangement view. For what we're doing, you will only need the arrangement view. The way to switch between the views is with the two circular buttons on the upper right. The vertical lined button is for the session and the horizontal lined button is for the arrangement. Just click on the horizontal lined button to get to the right view.
Adding your first song
Importing a song into Live is as easy as dragging and dropping the file from Windows file manager or OSX finder into an Audio Track in Live. In your new project, there should be 1 audio track and 1 midi track by default. You can delete the midi track now by clicking on it and hitting "delete" on your keyboard. Leave the audio track and drag your file in, placing the beginning on the first measure of the project.
Warping the song
After your song is imported and analyzed, you can set/fix the warp points and bpm of the track. This is the most important step of getting things to mix together properly. The way I do this is to start by deleting all warp points if there are any. Live generally doesn't quite get them right for actual songs, although it does a good job with small (under 10 second) samples, but since we're not doing that right now we'll want to tweak this part by hand. Drag the green warp point labeled "1" over to make the line under it match up with the exact start of the first beat of the song. This varies a bit from song to song because many don't start on a beat but you need to find the beat start regardless. Scroll through the song checking to see that the beat lines match up in that same spot on every beat. It needs to be extremely close to correct all the way to the end. Adjust the BPM field if you're seeing that the beats are getting further away from the lines. This may take a little trial and error to figure out. The longer the song is the harder this step is, as you lose precision toward the end.
The way I do this is to zoom in on a part, check it, adjust if necessary, zoom out, move maybe 30 seconds down, zoom in, repeat. If the part you're planning on mixing out of isn't dead on beat in that warp view, it will not work. You'll struggle to mix the song or end up just giving up. If a song seems inconsistent, you will have to create warp points to stretch or squeeze parts to make the some consistent. Just grab any beat and move it to do this. It's very difficult to explain how, so trial and error will be necessary to learn. The more you play with it the better you'll understand it.
Adding another song
To add and mix in another song, start by creating an audio track. This is done by right clicking anywhere in the blank space under the existing track controls and then clicking "add new audio track."
Drag and drop in your song like you did for the first, then set up the warp for the song, again, like in the first example. Once that is done, you need to determine how these two songs are going to mixed.
Mixing the songs together
Start by moving the song to where you'd like the mix to begin. It's important to remember that you don't even necessarily have to have the mix audibly start there. You have all the same options (and more) that you have with a regular mixer. You can fade the song in, start with no bass, leave it with no volume for the first x number of beats, etc.. There are a few controls that will become important now. One is the bright green number on the right of the controls for the track. That is the "track on/off" button. When green, you can hear the track, when dark, you can't. The button next to that with the "S" on it is the solo. When you have multiple tracks mixed and you only want to hear one, clicking that on will solo that track and mute all others.
Automating volume changes
Underneath the title of the track (mine is "1 Audio"), click on the pull down and select "Mixer." Underneath that, click on the pull down and select "Track Volume." A red line will appear across the whole track. This line is representative of the track's volume. Double clicking anywhere on it will make a dot that you can drag around to control the volume change. To delete a dot, just click on it once and hit the delete key. In my example, I made 2 dots, one to start the fade out and one to end it, then I dragged them into the spots I wanted them at.
Automating EQ changes
EQ Changes are similar to track volume changes except that you must use a plug-in. To add an EQ and automate it for a track, you must click the devices circular button on the left hand side of the screen, just under the triangle. Expand "Audio Effects" and click-and-drag the "EQ Three" from there into the track. A control for the EQ will appear in the bottom area. For that track, click under the track name (like for the volume) but instead of "Mixer", select "EQ Three." Under that you have several options. If you want to gradually change the EQ, use GainLo, GainMid or GainHi and use the same technique as audio volume for setting the dots on the red line. In my example, I use LowOn so that I can simple turn the bass off and on for the track. You'll see in the screenshot that the low line is where there is no bass and where it shoots straight up is where the bass turns on. This creates the effect during the mix of having no kick as the new song comes in.
Repeat until finished
Keep adding tracks and dropping new songs in, following the steps above until you have completed your mix. If you run into mixing problems, try using different songs and above all, always make sure your warp-sync is set correctly at the beginning of every single song you use. It's ok if you have to make little tweaks that make the end of the song a little off for the sake of a tight-sounding mix, but if you try to mess with the beginnings of the songs, you may end up having lots of problems several songs later down in your mix due to a cumulative effect.
repeat @ create new track until mix is finished