I know this is going to sound stupid, but my knowledge of a router is
mostly limited to putting a fancy edge on plaques.

I want to take a 30" length of 4" wide board and put a slot all the way
through the board about 4" long. the centerline of the slot will be 3/4
of and inch from the edge of the board. Where do I put the fence? Do I
hold the board above the router and slowly lower it in position, or do I
drill my 5/16 hole first into the board and fit the router bit into the
hole, bringing my fence up to the board? Will I get a true straight
clean slot this way, with no wobble?
Thanks for any and all answers!
Paul