I've Lost Count How Many Times I've Been Asked "How Do You Make A Mix?"
So Have Decided To Make A Very Basic Tutorial To Hopefully Show You What It's All About.

Before We Start, I'm Assuming You Have Software Installed.
I Shall Be Using Adobe Audition, Which You Can Get HERE If You Don't Already Have It.

OK.........Let's Go !!

When you open Audition you should be looking at the MULTITRACK SCREEN
(Don't panic if not, click the MULTITRACK TAB top left)

Right click on TRACK 1, and INSERT AUDIO, then choose a track from your collection with a beat that you want to use as your backing loop.
Use your ZOOM OPTIONS at the bottom to open out your track as you need to be very accurate when you make your loop.
If your loop isn't exact, nothing else will be!

Hightlight the section you want to use, drag in or out using the small YELLOW MARKERS at the top of the track
Click the LOOP BUTTON to play your loop, and adjust as needed to get the loop exact.

Once you are happy with your loop, right click on the highlighted part and CUT it.
You can delete the rest of the track your not going to use then right click and PASTE the loop section back into TRACK 1.
Right click on your loop and CONVERT TO UNIQUE COPY.
If you want to speed up or slow down your loop right click on it and click CLIP TIME STRETCH PROPERTIES. Tick the box to enable, and adjust the percentage accordingly (<100% faster, >100% slower). For the purposes of this tutorial i will be leaving my loop track "as is".
Right click again, and LOOP PROPERTIES. Then tick the box ENABLE LOOPING and click OK.
You should now have a LOOP ICON on the bottom left of your track, and by placing your cursor on the right hand side of the track you will now be able to extend your loop to whatever length you need.

I'm going to be using an acapella to run over my loop.
Follow the same steps as before and insert your acapella into TRACK 2.
The particular acapella i'm using has talking at the start, so i've simply highlighted this part and then right click DELETE. So i'm now left with the acapella thats begins right at the start of the vocals i want.

I Always adjust tracks manually, to get the best results i can, some software will "beat match" for you, but i find that results can vary a lot, and sometimes are not very accurate at all.
Ok, so we have our loop ready, and we have our acapella in track 2 trimmed to the start of the main vocals, and ready to match.Again, open out the tracks using your ZOOM CONTROLS so you can get the match as accurate as possible.
Move your acapella so the start of the wave matches the start of your loop exactly.
Right click your acapella and adjust accordingly using your CLIP TIME STRETCH PROPERTIES function. Alternatively you can right click, and drag out the end of your acapella to match the beats.
One thing to notice is that some tracks (and this happens a lot with "live" instrumentation rather than computer generated sounds) is that although you have matched the start and end, other parts of the track will not line up correctly.In this case i will cut out a small section of the track, paste in back in, and adjust only that part.... you will obviously need to do this for all of the track.
Once you have done that you should end up with something that looks like this.....

As you can see i've coloured the loop in red, and the acapella track is in the default colour green.
I've done multiple cut & stretches, and you can see how the acapella waves match up with the loop waves (not all of them will, as some will be on the half beat, but as long as your main ones are you should be fine!)
This is what that section will sound like.