Put the first across where you'd expect but in only 6 cells (read sentence 2 of the preamble to see what to put in the 6th cell). Put the first 4 downs in columns 1, 3, 4 and 6. The second across goes in 5 cells finishing at the end of the row.
Watch for double letters in cells - it helps to remember that as the grid is symmetrical every across in the first half of the acrosses has its counterpart in the second half at the end working backwards. Hope that helps to get you started.