All answers, both across and down, are part of a compound word. Take two of your answers from the across section to form a compound word and use for one of the across words. Take two of your down answers to form a compound word and use it for one of the down words.
Repeat until you've used all of the words in each section, across or down, and the crossword grid is filled in.
Each compound word will only be used once in the final grid.