I may or may not use a Significator - and how I choose it varies on mood. It can be deliberate or a random selection.
If I have a 'general question' - I may choose a Significator... mostly by matching as much as I can, the querent to the "personality" of a Court card (as I intuit it. Other people use Astrology to choose the Significator - or go by matching appearance of the card to the querent).
Then I'll use that card to perform Paul Foster Cases' "The 1st Operation".
Here, I shuffle the significator into the deck, split the deck into 4 piles - Earth, Air, Fire and Water. then find which pile the Significator has ended up in.
The Pile then tells me what area of the querents life is most relevant presently. The card before and after the significator represent "The Universes Message" to the querent - or I may decide to use them as recent past and future cards... etc.
If I have a specific question and am going to use the Celtic Cross - then whatever card I deal in position one - is the Significator.
If I want to use a Significator but don't want to remove the availability of that card from the complete 78 draw - I will use one of the spare cards that came with the deck as the Significator - or if there are none, choose one from a different deck.
If the Significator is the 9 of Cups - I would read that as the querent's present state: perhaps they have reached a position of achievement and satisfaction; are actually a bar tender; or they are the "go to person" that most people seek when looking for advice (like a counsellor)... or this person may be bordering on being sly and smug about something (i.e. a bit on the "pride before a fall" side)... etc. etc. etc.
@Calcifer - I was taught that if I was doing an absent reading, I should select a Court to represent the absent person - this would then mean that the deck was reduced to 77 cards - the no. of "The Will" - i.e. the reader is sort of "imposing their will" on them. Truthfully - I am uncertain what this means as I don't know the origin of the 77... never investigated it yet. Perhaps some more numerologically minded ATers can shed light on this here.
I was also taught to shuffle and randomly choose a Significator just before a reading to get some "insight" into the state of the querent/their state of mind etc.
I will do this sometimes.