how to write appendix

Tables, figures, or instruments that do not belong in the main text.
Items that might be included:

4.2 Environmental impact

each appendix must be labelled with a number (or letter) and title
the appendix numbers and titles must be listed on the Contents page under the heading Appendices (if more than one) or Appendix (if only one)
each appendix must be referred to by number (or letter) at the relevant point in the text.

The appendices follow the reference list.
They are lettered “Appendix A,” “Appendix B,” “Appendix C,” and so forth. If you have only one appendix, however, simply label it Appendix.
Put figures and tables in separate appendices. The appendix title serves as the title for a table if it is the only table in the appendix.
If you decide that certain figures and tables should appear in the same appendix, number them A1, A2, A3, and so forth, according to the appendix in which they appear.
The materials in the appendix must not extend beyond the margins of the rest of the paper: Reduce the appendix materials as needed.

APA 7 addresses appendices and supplemental materials in Section 2.14 and on page 41:

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