Recently did this on my bench. I had routed the mortises that those
stretchers go into. I drilled from the inside of the mortise through
the leg. Used a backing board to prevent blow out on the leg.

Then I inserted the stretcher into the mortise and used the existing
hole as the drill guide. Had to make two hardware store trips though,
as I went to a store that I know is less expensive for these larger
bolts. SOMEONE put 5" bolts in the 4" bin and I didn't measure them in
the store...

Alan