Literal translation
将欲翕之,To shrink it, If you want to shrink it,
必固张之; Expand it first. Must first expand it.
将欲 取之,To conquer it, To take over it,
必固与之 Be with it first. Must strengthen with it (ie with friendship etc)
The last two lines can also be interpreted as "if you want to take it, you must first give yourself to it"