I’ll demonstrate the “choose newest” cell selection method, which will hopefully be enlightening enough that you can imagine on your own how the “choose random” method might proceed.
The algorithm begins by adding an arbitrary cell to the list. We mark the cell as “in”; I’m also going to assign the cell a number, visually, to help keep track of the relative age of each cell. Also, cells in red are in the list of “live” cells; they’ll go white once they’ve been removed from the list.