Theory for the series finale.

At the end of Season 8 or the beginning of Season 9, we first see the mother at the wedding and Ted meets her officially. The Season 9 opening is changed to pictures of all 6 of them at MacLaren's. Season 9 shows us Ted's self-proclaimed "greatest love story ever". Him and the Mother dating and falling in love and his proposal.

The 3rd to last episode is the day of Ted and the Mother's wedding and Ted sets up a few very interesting stories about that day (as we have seen many times before, i.e. The Goat, The Wedding Bride,etc.) Then at the end of the episode the Mother walks in and tells him to hold on a minute.

2nd to last episode - The Mother explains the entire story from her perspective (Losing her umbrella, the bar on St. Patrick's, sitting in Ted's class, etc.) So we see how all those puzzle pieces we've been given over the years fit together. And since she isn't as long-winded as Ted, it can be fit into one episode.

Finale - Ted and the Mother jointly explain their wedding day and conclude those stories. They bring back the most memorable characters from the whole show (Ranjit, The Captain, etc.) as guests at the wedding. After they finish telling about the wedding they give a brief summary of what's happened since with Barney, Robin, Lily, and Marshall, etc. And it ends with a pan around the room of everything important in the show(the yellow umbrella, a bass guitar, etc.) and ends with Ted and Mother sitting on the opposite couch from their kids.