Biscuit joinery without a biscuit joiner

Cutting biscuit slots

Cut slots for biscuits with a slot cutter mounted on an arbor that's equipped with a bearing. Cut a continuous slot on boards thicker than 3/4 in.

If you want to make biscuit joints, you don't have to buy a biscuit joiner. In most cases, a router equipped with a 5/32-in. slot bit can cut perfect slots to fit the biscuits. Mark the biscuit positions on both adjoining boards as you would with a biscuit joiner. Then cut a slot that's about 1/2 in. longer than the biscuit. On thick boards, you don't even have to mark out anything; just cut one quick, continuous slot on each board. Add glue and biscuits and then clamp it to create a strong joint.
There are two situations where a router can't substitute for a biscuit joiner: A router can cut slots only along the edges of a board, not across its face; and it can only cut along square edges, not beveled ones. Most slot cutters cut slots about 1/2 in. deep, which suits No. 20 biscuits. If you want to use smaller biscuits, buy a kit that includes three bearing sizes for No. 0, No. 10 and No. 20 biscuits (available online and at woodworking stores).

source: http://www.familyhandyman.com/tools/routers/advanced-router-techniques/view-all