I made a sample sequence diagramAtom with PlantUML. Most frequently used rules are in this sample, so you should be able to make your own diagrams by modifying this sample if you forget the rules.
' Writes comments after singl quotaion mark. @startuml ' Color Setting skinparam actorBorderColor black skinparam sequenceParticipantBorderColor black skinparam sequenceLifeLineBorderColor black skinparam sequenceArrowColor black skinparam noteBorderColor black ' Declaire participants. ' You can define an alias usibg "as". actor Actor as A participant Participant as P A -> P : Synchronous Message activate P A <<-- P : Return Message deactivate P A ->> P : Asynchronous Message A -> A : Message to Self ' Note (For messages) A -> P : Message note right Note: note (left|right) ... end note (Or just "note (left|right) : ...") end note ' Note (For participants) note over A, P : note (left|rigght|over) of ' Text Decoration A -> P : Multi Line\n(Line Break) A -[#blue]> P : <font color="blue"><b>Change Style</b></font> ' Conditional Branch alt guard condition A -> P : Message else guard condition A -> P : Message end ' "Message to Self" doesn't work with "Activate" well. A ->> P : Asynchronous Message activate P P -> P : Message to Self deactivate P @enduml