My daughter needed a "big girl" bed and I loved the look of an upholstered bed. Building it seemed our only option since all the inspiration I looked at had $$$$$ price tags.
Supplies:
3/4" plywood for platform
1/2" plywood for headboard
1x4 for crossboards
2x2 for middle crossboard support
1x2 for side crossboard support
4 legs, stained and sealed
4 top plates, to screw legs into base of bed
wood screws
batting, HIGH LOFT and as many layers as you can
3.5 yards fabric
nailhead trim and rubber mallet

Cut your 3/4" plywood into 9" widths, 2 at 38.5" and 2 at 75.5" Using wood screws, screw together with the longer sides on the outsides of the short sides so the final measurements equal 39x75. Then, add your 2x2 and 1x2 supports to the inside of the frame about 2" below the top of the frame and then add the 1x4 crossboards.

Upholster with batting, muslin, and your final fabric

Add your beautiful stained legs

Upholster the headboard, also with batting, muslin, and fabric

Add nailhead trim according to directions

Sew some easy envelope pillows and you've got a very chic twin-sized bed for cheap!

You can read more about the supplies I used on the blog post, as well as more information about my inspiration!