Seam allowance is included with all pattern pieces.
If you're using regular fabric, you'll need to bond it with heavy weight interfacing to get the required stiffness. I recommend Craft-bond interfacing. It's an iron-on material you can find at most fabric stores.
Trim 1/4" from the edge of your interfacing before bonding it to your fabric pieces, to keep it from showing on the edges.
If you're using craft felt, you won't need to worry about the interfacing. Craft felt is a great option - it's very inexpensive and comes in a huge variety of colors. It also feels cartoony and costume-y to me.
Print and cut out your pattern pieces. If you're making the Plague Doctor, tape the two pattern pieces together to make one long piece.
Lay your pattern on top of your fabric. If you're making the large beak or the Plague Doctor, place the fold line along a folded edge of your fabric so you can cut through both layers at once.
Trace around your pattern with a marker, or pin the pattern to the fabric. Cut along the edges - seam allowance is already included.
These beaks can attach to any mask, or you can make a mask base specifically for the costume mask. For the larger patterns, it's recommended to use this mask base since it's a bit larger and secures to your head strongly.
Cut one piece out of your fabric and one out of your lining on the fold.
Stitch along the upper and lower darts as shown. This will make the mask fit to your face.
With right sides together, stitch and/or serge along the top and bottom edges. Turn and press. Finish finish the sides and add a nose wire and ribbons or elastic to tie behind your head or loop over your ears.
More detailed instructions for making a full mask base can be found in this Mask Tutorial.
For this example I'm keeping it simple with craft felt. If you want a double layer with interfacing, follow the directions for the Large Beak - it assembles the same.
Find the middle of the lower beak and pin it to the top center of your mask, with the beak facing upwards as shown. Pin the two corners near the ear holes. Add a couple more pins to mimic the entire seam line.
Repeat with the upper beak. Flip the beak down along the pin line so it forms a beak. Try your mask on. Adjust the pins as necessary until you're happy with the placement.
For this mask, we'll use two layers of fabric for each beak part, with a layer of interfacing bonded to one of the fabric pieces on the top and bottom.
Trim 1/4" from the edge of your interfacing before ironing it on to your fabric. This will keep the seams from getting too bulky. Bond your interfacing to your fabric according to the directions.
With right sides together, stitch or serge the sides and front edges, leaving the back edge open. Turn and press. Baste the raw edges together.
Attach them to the mask with pins in the same manner as used for the Small Beak above.
You can sew this to an existing mask or use the mask base from the Large Beak pattern (recommended). Prep the mask base the same as for the Large Beak.
Trim your interfacing by 1/4 inch on all sides to keep the seams from getting bulky.
Bond a piece of interfacing to both of the fabric pieces. You'll want pretty stiff fabric so the beak stands up as much as possible.
With right sides together, stitch along the seamlines on both sides, leaving the short edge open.
Stitch a second time over the first seamline. Trim the seams close to the stitch line. Trim the point as close to the stitching as possible -- the less bulk you have in the point the pointier it will be. Turn right-side out. I found it helpful to jam a chopstick way down into the point to really push it out as far as possible.
Sew a narrow hem around the opening. We'll need to stitch this to the mask from the outside, since it's a closed design, so make the edge look neat.
My fabric wasn't quite stiff enough even with the interfacing, so I stuffed a bit of batting inside to help the beak stand up and look smooth. Don't stuff too much or it will interfere with your breathing!
Pin the beak to your mask base from the outside, going all the way around and adjusting until you like the placement. Stitch it to the mask topstitch-style, through all layers.
This beak is pretty heavy, so be sure you have a really strong cord or ribbon to attach it to your head. I ran a leather cord through each of my mask's edges, then tied both the top strings and the bottom strings separately behind my head to keep this behemoth in place.