If you use the correct bit you can plunge down to get started.

Set the fence with a pencil mark of where you wnat to start and a stop
block for the finish if you need real precise.

You will get a little bit of shake because the bit is cutting in two
directions. It would be nice to have some hold downs but a little
difficult when you are plunging, but make some sort of feather board,
springs of even just a second fence to help keep it from jumping.

a 5/16 slot should be able to cut in one pass in most woods pretty
easy but play with feed speed. Also, clogging can be an issue where
the chips don't evacuate but a full through slot should make this not
to problematic and strong down suction dust removal will be a big
help.

Another option is to place the piece on top of a sacrificial piece of
ply and construct a jig over the top with a few boards and do it free
hand. In this case I would take multiple succesivly deeper passes and
you can control super precise start and stop locations.

Or even just use a base guide on the router and do it free hand.

Lots o options

You will get a little shake because the bit is cutting in two
directions when it is in
On Mar 18, 9:29*pm, sam wrote:
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