Create a MIDI channel by clicking the "Create" menu on the top toolbar and selecting "Insert MIDI Track." Alternately, this can be accomplished by pressing Ctrl, Shift and T on a Windows machine, or Command, Shift and T on a Mac.
Drag and drop a sampler or drum machine from the left menu onto the space at the bottom of the screen. The standard Ableton drum machine can be found by clicking the "Live Device Browser" button in the top-left corner, and selecting "Drum Rack" from "Instruments." Alternatively, you can click on the "Plug-In Device Browser" button, and select any third-party drum machine and sampler plug-in that you have installed yourself.
Drag and drop your chosen drum samples onto the blank sample pads and locate the MIDI note that triggers your snare drum. You can do this with a connected MIDI keyboard, or by using the keys on your computer keyboard. The central row of keys, starting "A, S, D" are the white keys, with the black keys in the appropriate spaces on the row above. The "Z" and "X" keys allow you to raise and lower the target octave.
Double-click a clip slot in the upper portion of the channel strip to create a MIDI clip. The space in the lower portion of the view will switch to display a standard MIDI grid instead of your drum machine. You can switch back to the drum machine at any time by clicking the channel's title bar.
Create a snare hit by double-clicking in the MIDI grid area on the row to which your snare drum is assigned. Select the colored block that appears and hold down the Alt key. Drag and drop a copy into the adjacent grid space and repeat, until you have a roll that lasts as long as you want. The lollipop-shaped rods at the bottom of the grid represent the velocity of each hit. Ramp them up from low to high by clicking and dragging on them to create a roll that gradually increases in velocity. This a popular way to "announce" the next section of a song.