You need four items, 2 for Tidus-Sun Crest and Sun Sigil and 2 for Yuna-Moon Crest and Moon Sigil and you get these by;
Sun Crest-At Zanarkand Dome-The Beyond; go down the stairs in the north.You will reach another room containing a chest with the Sun Crest.
Sun Sigil-complete the catcher chocobo race in the calm lands in a total time of under zero seconds.
Moon Crest-Besaid Beach-go into the water and go to your right to a little area with there is a chest with the moon crest.
Moon Sigil-Remiem Temple-you must have all eight aeons and defeat all of Belgemine's aeons and send Belgemine to the farplane.
P.S-You must have the cloudy mirror by racing the chocobo at Remiem Temple and beat him the do the little mission where you find the women and boys dad/husband and then find the boy at macalania woods then use the cloudy mirror to make it the celestial mirror and thats the place you go to upgrade your weapons when you have the right items and weapons
If you need help with Kimarhi's,Rikku's,Auron's,Lulu's and Wakka's just ask