To cite the current, hierarchical ctsem use:

  Charles C. Driver, Manuel C. Voelkle (2018). Hierarchical Bayesian
  Continuous Time Dynamic Modeling. Psychological Methods, 23(4),
  774-799. 10.1037/met0000168

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Hierarchical {{Bayesian}} Continuous Time Dynamic Modeling.},
    author = {Charles C. Driver and Manuel C. Voelkle},
    journal = {Psychological Methods},
    year = {2018},
    volume = {23},
    number = {4},
    pages = {774-799},
    doi = {10.1037/met0000168},
  }

To cite the original ctsem use:

  Charles C. Driver, Johan H. L. Oud, Manuel C. Voelkle (2017).
  Continuous Time Structural Equation Modeling with R Package ctsem.
  Journal of Statistical Software, 77(5), 1-35.
  doi:10.18637/jss.v077.i05

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Continuous Time Structural Equation Modeling with {R} Package {ctsem}},
    author = {Charles C. Driver and Johan H. L. Oud and Manuel C. Voelkle},
    journal = {Journal of Statistical Software},
    year = {2017},
    volume = {77},
    number = {5},
    pages = {1--35},
    doi = {10.18637/jss.v077.i05},
  }