You can try this in the body of the procedure, assuming that the list has at least three elements:
(cons (first ls) ; take 1st element
(cons (second ls) ; together with 2nd element
(rest (rest (rest ls))))) ; grab the rest starting from 4th
The above will work in Racket. But if you prefer to use more standard procedures:
(cons (car ls) ; take 1st element
(cons (cadr ls) ; together with 2nd element
(cdddr ls))) ; grab the rest starting from 4th