This easy (and quick!) waterproof 'raincoat' for bicycle seats beats
bikes wearing grocery bags as rain covers, and definitely tops sitting
on a squishy-just-been-rained-on seat. A perfect gift for the
commuting cyclist, the bike enthusiast, or just someone who you know
would love their bike to look that much more pretty.

Materials:
1 yard oilcloth, or other waterproof material
1 17-inch length of 1/4" wide elastic
1 spool matching thread
1 safety pin
a pen, pencil, or tailor's chalk
a ruler
scissors/rotary cutter
any other embellishments you want to add (embroidery floss, patches, etc).

(note: this tutorial was also featured in Venus Magazine's December 2006 Issue)
(also note: this is an entry in the Etsy & Instructables Sew Userful Contest. You can see my etsy listing over here)

Step 1: Creating the Seat Template

Loosely trace the top portion of your bicycle seat onto a sheet of plain or
tracing paper - this will serve as the template for the top of your
rain cover. You don't want this to be an exact tracing of the top, but
more of an outline. It helps if you try to keep at least a little
symmetry on both sides of the seat.

Step 2: Expand Your Template

Increase the size of the tracing you just made by about 1/4" all of
the way around (and smooth lines where necessary to make the outline a
little bit more 'bike-seaty' - everybody knows it's hard to trace 3-d
objects!

Step 3: Cutting Your Waterproof Material

Cut the template out of the tracing paper (including that extra
1/4") and outline the pattern onto the vinyl (or other waterproof
material you are using) using a pen or pencil (or tailor's chalk if
you want to get really fancy). At the same time, mark off a 34" X 4.5"
rectangular strip on the vinyl as well. This will be the base of your
cover. Once marked, cut out both pieces using scissors or a rotary cutter.

Step 4: Creating the Elasticized Bottom

Take the 34" X 4.5" rectangle and fold a 1/2" seam down the long
side (so your rectangle will now be 34" X 4" with one folded up edge)
- wrong sides together. Secure this fold by sewing a 3/8" seam on the
fold (3/8 from the folded edge).

Feed one end of the elastic length onto the safety pin, and then
guide the safety pin (with attached elastic) into the folded tube/seam
you just created.

Using the head of the safety pin, feed the pin and elastic with
your fingers until the elastic is slightly more than mid-way through
the folded and sewn tube (about 18 - 20"). When the pin has reached
this point, secure the elastic at the beginning of the tube (back at
the point where you fed the safety pin into the tube) by stitching
over the elastic & the vinyl. This means that the elastic isn't going
*anywhere*. Continue feeding the elastic through to the other end of
the tube.

By this point, your rectangle will have become slightly curly (as
the elastic is forced to stretch to reach the length of the tube. You
want the elastic to be able to *stretch* to the full length of the
rectangle. Once you can do this, secure the elastic at the other end
of the tube in the same way you did in step #6.

Trim elastic ends and sew both short ends of the rectangle together
- right sides together. Now you have the stretchy loop for the base!

Step 5: Sew the Bottom to the Top

With right sides together, sew the unfinished edge of the "seat"
cutout to the unfinished edge of the rectangle with a 1/4" seam (put the right sides together);
you'll have to follow some curves - this will be the toughest sewing
part.

Be sure to backstitch at the beginning and end of your stitches.

Once you've sewn all the way around the circumference of the
seat, you're done!

Step 6: Trim, Tidy & Embellish

Trim & tidy threads and edges, embellish as required (or desired),
and try it onto the bike for size (and style!).

This instructables tutorial is part of the Etsy & Instructables contest. To see the finished product, view this item in my etsy shop online here: http://www.etsy.com/view_listing.php?listing_id=6055270