For the hair (easy adaptation of a devil wig. the horns were sown down.)
The upper body: Since we live in WA state and it is cold here I bought a white turtle neck and dyed it flesh tone. Then using purple jersey fabric I cut and hand sewed the scallop shells to the bust (making sure they fell in the right area on my 3 year old) To finish off the top I cut a small strap for the "bra" and sewed that in place.
For the tail I cut 2 arrow shapes of satin like material (it looked more fishy). Then I cut 4 triangles out of green jersey material and made little "pillows" out of them for the tail fins. I attached one to each side of the arrows' point (2 on each arrow) Then I sewed the arrows together in a tube with the points together. After this I added some elastic to the waist so that it would fit my little one and be used for dress up later on.
The hair: Santa wig
"Chest": thermal top dyed skin tone. for the abs and pecs(not shown but there) I painted them on using leftover dye and a paint brush.
the tail was created in the same way as the little mermaids using blue jersey fabric.
Trident: Devils pitch fork spray painted gold.not pictured above (but way above they are)
Crown: milk jug. I drew out the general shape of King Trident's crown cut it out and spray painted it gold
Wrist guards: plastic large fast food cups. These were embellished with hot glue. The bottom was cut out for his hands and then the whole thing was spray painted gold.
Lastly Ursula (please excuse the cig I have since quit [half a year and counting])
Purple top is a thermal dyed lavender. The black section was done in two parts. The top half was cut to mimic the bust of Ursula and then hand sewn on to the purple top. On the skirt/tentacle section: I cut out 6 triangles of purple jersey (figured with 2 legs I would have 8 - and the Ursula in the movie only has 6 legs) then a long section of black that I cut triangular strips into leaving a 3 inch band at the top to attach it to the shirt section. Then I sewed the purple triangles to the black ones leaving a hole to place polyfill and a wire coat hanger in. Once I had done put in the wire and stuffing I stitched the hole shut and sewed the "skirt" to the top. In hindsight it could have been easier to get into and out of but it worked.
Nails: plastic fake nails cut into points and painted red.
earrings: old dangly earrings with the bauble cut off and a purple painted shell hot glued to the chain.
Necklace: a piece of the black fabric used for the costume cut to fit with a yellow painted shell hot glued to the band and velcro for the "clasp" at back
Wig: Don King wig spray painted white (the spray paint made it stand up like ursula's)
face paint (white costume paint tinted with purple eyeshadow, for the face. For the eyes black costume makeup)