The entry method is as follows
e.g. 15 Across TRANSCENDS + 6 Down DISC CAMERA share the letters SC, so enter those two letters in the one cell at Row 3 Column 9.
Last 3 letters of 15 Across are entered as single letters, but the first 6 are entered as 3 pairs. TR / AN / SC
First 2 letters of 6 Down are entered as single letters, but the last 8 are entered as 4 pairs SC / CA / ME / RA
The double letter entries can also occur in unchecked cells, but arriving at how this works depends on word-lengths and thir occurrence in a specific area of the grid. If it helps, NONE of the 6 words to be highlighted include any double letter cells.
The extra words in the down clues include QUICK, JAWBONES, LETCH, CORK, ALDER among others. Using T O E S A variously to correct them to give synonyms for duck sound, butcher, sucker, prepare food, change should show a thematic connection which tells you what to do to & with the title to give you the theme.