In general it's not a good idea to stub methods for the object under test. Though, since you asked about the syntax, what you are looking for is RSpec Mocks:
describe Survey do
subject(:survey) { Survey.new(...) }
context 'requesting the description' do
it 'contains product name when it has input' do
survey.stub(selected_input: 'apples')
expect(survey.description).to eq "How's them apples?"
end
end
end