You can register your sequence as default sequence on sequencer where you want to execute.
uvm_config_db #(uvm_object_wrapper) :: set (this,"my_env_inst.my_vir_seqr_h.main_phase",
"default_sequence",my_seq:: get_type());
You require to use `uvm_declare_p_sequencer macro if you are accessing members/variables of sequencer from sequence itself. Either you use start method or register method , you would require this macro.