Read books, read lots of books, and then read more. You will absorb how others before you have written them.
Write down something every day in a notebook (or most days, if you can't manage every day) Write anecdotes, quotes, musings, references to other stories connected with yours.
When you are ready to start, make a list of chapter headings that follow a recognisable "journey" through time, or through the progress of your parents' dementia, and shuffle the "material" in your notebook into appropriate chapters. Expand them, add to them, and refer back to them in other chapters.
There are lots of helpful ideas here, and suggestions for finding more help.
Good luck.