The shared resource is represented by the token in P2. P2 will be named as "Resource available".
P1 will be named as "Resource alocated to activity 1" and P3 will be named as "Resource alocated to activity 2".
Transitions T1 and T2 represent the start and the end of activity 1 and T3 and T4 represent the start and the end of activity 2.
After entering the simulation mode by activating the simulation mode the workspace is changed - a list of saved reached markings is displayed, containing already the initial marking:
From now on, all active transitions will green. All inactive transitions will be red.
To fire a transition, just clic on it. When you clic on T1, the net reaches the following marking:
New net marking after firing T1
Notice that only T2 is active now. When you fire it, the initial marking is reached. You can also clic on the "Save marking" menu/button to add this particular marking to the "Reached markings" list:
"Reached markings" list
If we had clicked on T3 instead of T1, the marking reached would be:
New net marking after firing T3
Notice that now only T4 is active. When firing it the net returns to the initial marking.