timing is very important.
basically fit the whole lot including the belt but leave the cam wheels undone.
you will need a timing pin for the zetec which is put into the 10mm bolt hole at the front of the block.
a plate goes into the end of the cams and then you nip up the cam wheels.
you can do it by eye but its not so accurate.
when pistons 1 and 4 are fully up at 'top dead center' then you line the end of the cams up and tighten them up.