Go into "Aircraft", then "Edit [aircraft name]".
Under the Physics tab, for each of the parts you want to break more easily, look for the "Strength Multiplier(%)" under the Advanced Parameters for that part. Note that there may be both top level components (like Fuselage) and sub level components under that level (Landing Gear) that have their own settings. Reduce the value as you see fit, until it breaks as often or as easily as you want.
Note that for stock planes, you will have to save your changes as a copy with a new name.
Of course, also go into "Simulation", "Flight Failures" to set the Frequency and to Enable what to fail, if you also want to experience them on rather random occasions, and not just when you do something wrong.
For fun, hit the "~" key, and at the command line type "showmethemoney" and hit enter. Close the box, and go crash.....