Whether you call it a Bowtie, a Butterfly or a dutchman (or many other names) it still does the same work to stabilize a crack in the woodwork or just to look good. this is a simple task that a lot of op people over think and it can take far less time than you think.
Step 1: Draw Out the Bow Tie
Some people have deep formulas for this or extensive patterns. The only important thing is that the middle is smaller than the two ends. You can honistly make it almost any shape you would like.
I like mine where one end is longer than the other and the two center points do not line up. I just use a straight edge to draw it out until it looks good to my eye. Do not over think this step. This is where you can have some fun and be a bit creative. Straight lines are easier, but you do not have to stay there.
Step 2: Cut the Bow Tie
I like to cut down to the center points on both sides then use a rip saw to come in from the corners to the waste of the shape. I often find the saw left a bit ragged of an edge and like to clean it up with a chisel.
Step 3: Mark Out the Bowtie
You can do this freehand but you have to be very careful that the Bow Tie does not move. come people use salt to help it stay in place, but my tool of choice is Carpet tape. The cheaper the better so that it comes up easier.
With the butterfly set where you want it, use a marking knife to trace out the shape. I like to undercut the block slightly but at first, just focus on transferring the line directly. This can be done with a single sided bevel so that the flat side rides along the knife.
Step 4: Remove the Waste
Now with a larger (SHARP) chisel place the flat side toward the line and chop down one or two strikes. Stay away from the marking line 1/16" or so. save that line for the very end.
Next, I come in with a smaller chisel bevel down and remove 1/16" - 1/8" layer. This should take you down to the depth of the chopped line.
Then repeat the prosses. chop in with a stop cut and remove the waste. this will take you down a bit each time. I like to go down 2/3 the thickness of the board but some like to go all the way through.
Step 5: Clean Out the Hole
I like to have a flat bottom for the glue to better adhere to the Bowtie. I do this with a Hand router . It will give you a clean surface ready for glue.
Last I put the chisel right into the marking line and keep the chisel vertical. I will slowly chop all the way down making sure to not move that line at all.
At this point, you can test the butterfly fit. but is should be so tight that it is almost impossible to get out. So don't shove it in or you might not get to glue it.
Step 6: Install the Butterfly
Glue is a good thing here. I spread a good amount on the bottom and around the edge. then drive it in place. you should see glue squeeze up around it on all sides.
Step 7: Trim It Off Flush
Last it is time to trim it flush with the surface.
I use a chisel to bring it close to the surface. be careful here. If the grain is not straight in the Bow tie it might dive below the surface. make sure to work with the grain.
Then I will use a plane to take it down to the surface of the wood. but the best thing to actually smooth it out is a card scraper. as the grain will be going in two different directions the card scraper can cut across that. now it is ready for finish!