1
I planted a sunflower seed.
To express this, sign 'sunflower' and 'plant', then transition into the sign for 'seed'. Hold your non-dominant hand in an 'F' handshape to represent the small object, and clearly fingerspell S-E-E-D with your dominant hand to specify what you planted.
2
The bird ate the seed.
Start by signing 'bird' near your mouth, followed by 'eat'. Then, use your non-dominant hand to form the 'F' handshape classifier, showing the size of the seed, while your dominant hand fingerspells S-E-E-D to complete the sentence.
3
We need to buy grass seed.
Begin with the signs for 'we', 'need', and 'buy'. Next, sign 'grass' by pushing the heel of your open hand up under your chin. Finally, hold your non-dominant 'F' handshape and fingerspell S-E-E-D with your dominant hand.