In order to make this easy to follow, I will not describe the best way to count beats, etc, but I think you'll get it easily by practising, so here we go.
1. Searching for the song
This is the most important part, you can find the song you want to sample after hearing it on a nostalgia radio or after searching for a song for 2 hours, but however, you must have the audio file of the song you want to use for your instrumental.
2. Let's do it
Run FL STUDIO (i'm using the 9th version) then go to the mixer, look for "Edison" in the effects browser and click on it.

Here's what Thomas Edison looks like:

Now look for your audio file, drag and drop it onto Edison (which is found in FL STUDIO):

Select the loop mode, then select the part you want to sample on the waveforms, then play. It's going to loop, you'll judge by yourself if it's corresponding to what you want. You can zoom and dezoom (PageUp and PageDown keys) to get more precision.

After that, push CNTRL+C to copy that part. Re-open the Mixer (1st picture, button n°1), then, in a new insert (Master, insert1, insert2, insert3... those are inserts) add an Edison Editor as the way you did it the first time.
Then, push CNTRL+V to paste that part on the new Edison that you opened:

Save it. To have it in your sounds browser (the sound library in the left), save it in:
(Image-line Directory)/FL STUDIO (9 or 8 or etc...)/Data/Patches.
You can create your own folder under "Patches" where your futur samples are going to, in order to get it organized.

Now your file is there. I'll show you the way to use it and some tips to set it.
Open a new project (under FL STUDIO), set the tempo/BPM to "90" and insert the file you just created on the pattern.
Fill the first bloc, then press play and count(and remember :p) the number of beats/bars that the sample approximatively "consumes" before it finishes playing. You can follow the little orange rectangle which moves when you play a pattern. This image will show you the way to do it.

You counted? Okay, so now click on the sample to pop-up a new settings windows, and set it like so. (The time settings may vary depending on the beats you counted)

And that's it! Before I finish the tutorial, here's a tip to control it like an instrument:

DONE!! Now you have it, you can set the tempo as the way you want to, then compose your isntrumental by adding the other elements (drums, instruments, etc...), setting the FX, etc...
I hope that you enjoyed the tutorial!
Leave your thoughts!