How do you make a homemade shinigami outfit?

Answer 1

The top "robe" is actually two, there is a white under layer and then a black over layer. This can be clearly seen when Ishida is dressing in the Shinigami uniform. The pants are called the hakama.
You can do a search on patterns to make kosode (the robes for the top portion) and hakama. The socks are called tabi. You can just cut a split in a pair of white cotton socks for these. The shoes are called waraji. There are also "how to" pages out there.

So in short look for how to make lists for these items:
kosode
hakama
tabi
waraji

P.S. zori do not have the wrap around straps that the shinigami are seen wearing. The zori are a different type of shoe like the Americans got loafers and sneakers.

Answer 2

simple

by owning a death note

but you can only get one if a shinigami drops it, which they only do every thousand years

not so simple, then...