Step 1: type: sv_cheats 1 into the console
Step 2: type: bind o "ent_fire !picker setbuilder !your tf2 user name in this space" (you may want to copy + paste this command since its hard to remember)
Step 3: type ent_create obj_sentrygun (this will spawn blu lv 1 sentry gun that you can dmg but it will shoot you(unless in MVM))
Step 4: press o while looking at the sentry
Step 5: pick the sentry up (right click)
Step 6: put the sentry down
Step 7: repeat as many times as desired