In order to slice a drumloop to a mapped instrument, do the following.
First load in the drumloop sample.
Make a selection that you want your first slice to be, and trim it. The sample will now contain that slice.
Repeat the steps until you have the entire drumloop splitted into several samples.
If you press New Sample, it can ask you if you want to create an instrument with it. Press no or you'll get unwanted instruments.
Head over to the instrument tab and create a new instrument. If this is your first instrument, it'll ask you if you want to convert all samples to instruments first. Press no or you'll get unwanted instruments.
You'll now have an empty instrument. There are several ways to make sample mappings. You could use the keyboard function  for it, but you can also type directly into the overview box (white with notes and ..)
Its far easier to type it in, so we're going to do that.
Click in this white list that should say something like:
If you want speciffic mappings to multiple notes you can do that by changing the octave. If you just want to have something simple for now, scroll down to C-4 (Octave 4 or 5 are usually the default octave)
Move to the right of C-4, and press Q.
It will add C-4 01 (where C-4 is the note, 01 the samplenr)
Change anything until you have the right sample and note, and then move down to the next. For drumloop slicing, I recommend to only use whole notes unless you have more than 10 samples.
So your next note would be 2 down (D-4 ..)
Press Q again, but now change the samplenr to 02 (or whatever the nr of the new sample is)
Continue like this until you've mapped your entire breakbeat.
In addition you can also move to range C-3 and do the same but with Z (note C-3) so it will be a slowmotion kind of breakbeat and you can choose.
Once you have your breakbeat finished, you can save the instrument as an .iti and next time you can just load in this .iti to have the entire breakbeat with sliced settings ready.
Note: for this small tutorial, I've used a basic keypreset that has only 2 octave ranges (not the default which has 4)
In order to get this, you can load amongst US_lpchip_IT.mkb key preset (which has all the keys Impulse Tracker has).
I hope this helps. If you have any questions, feel free to ask.