What's missing while trying to verify signature of a checksum?

THIS IS WHAT I HAVE SO FAR:

Code:
I.) source: https://www.debian.org/CD/verify

pub 4096R/64E6EA7D 2009-10-03
Key fingerprint = 1046 0DAD 7616 5AD8 1FBC 0CE9 9880 21A9 64E6 EA7D
uid Debian CD signing key <debian-cd@lists.debian.org>

pub 4096R/6294BE9B 2011-01-05
Key fingerprint = DF9B 9C49 EAA9 2984 3258 9D76 DA87 E80D 6294 BE9B
uid Debian CD signing key <debian-cd@lists.debian.org>
sub 4096R/11CD9819 2011-01-05

pub 4096R/09EA8AC3 2014-04-15
Key fingerprint = F41D 3034 2F35 4669 5F65 C669 4246 8F40 09EA 8AC3
uid Debian Testing CDs Automatic Signing Key <debian-cd@lists.debian.org>
sub 4096R/6BD05CFB 2014-04-15

II.) sha512sums.sign

III.) created my own private key pairs

IV.) know how to sign a .txt file and verify it hasn't been modified

V.) checksum for the .iso file

VI.) the .iso fileI tried to download debian keys:
Code:gpg --keyserver keyring.debian.org --recv-keys 0x673A03E4C1DB921F
gpg: WARNING: unsafe ownership on homedir '/home/b/.gnupg'
gpg: failed to create temporary file '/home/b/.gnupg/.#lk0x018e3900.1937': Permission denied
gpg: keyblock resource '/home/b/.gnupg/pubring.kbx': Permission denied
gpg: failed to create temporary file '/home/b/.gnupg/.#lk0x018e5d10.1937': Permission denied
gpg: connecting dirmngr at '/run/user/1000/gnupg/S.dirmngr' failed: Permission denied
gpg: keyserver receive failed: No dirmngr