Create CA key and cert

# openssl genrsa -out server_rootCA.key 2048
# openssl req -x509 -new -nodes -key server_rootCA.key -sha256 -days 3650 -out server_rootCA.pem

Create server_rootCA.csr.cnf

# server_rootCA.csr.cnf
[req]
default_bits = 2048
prompt = no
default_md = sha256
distinguished_name = dn

[dn]
C=DE
ST=Berlin
L=NeuKoelln
O=Weisestrasse
OU=local_RootCA
emailAddress=ikke@server.berlin
CN = server.berlin

Create v3.ext configuration file

# v3.ext
authorityKeyIdentifier=keyid,issuer
basicConstraints=CA:FALSE
keyUsage = digitalSignature, nonRepudiation, keyEncipherment, dataEncipherment
subjectAltName = @alt_names

[alt_names]
DNS.1 = server.berlin

Create server key

# openssl req -new -sha256 -nodes -out server.csr -newkey rsa:2048 -keyout server.key -config <( cat server_rootCA.csr.cnf )

Create server cert

# openssl x509 -req -in server.csr -CA server_rootCA.pem -CAkey server_rootCA.key -CAcreateserial -out server.crt -days 3650 -sha256 -extfile v3.ext

Add cert and key to Apache2 site-file, HTTPS (port 443) section

SSLCertificateFile    /etc/apache2/ssl/server.crt
SSLCertificateKeyFile    /etc/apache2/ssl/server.key

Copy server_rootCA.pem from the server to your machine..

# scp you@server.berlin:~/server_rootCA.pem .

Chromium -> Setting -> (Advanced) Manage Certificates -> Import -> 'server_rootCA.pem'

YOU ARE ALL DONE!

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P.S. Instead of creating a functional CA & server cert pair (per the instructions above) you could simply disable HSTS headers in your HTTP server config.
This will prevent Chromium from enforcing HTTPS and will allow users to click “Advanced → proceed to your.url (unsafe)” without having to obtain and install your custom CA (server_rootCA.pem) certificate. In other words – having to disable HSTS will allow your site to be publicly viewed over HTTP and/or insecure HTTPS connection (beware!).

For Apache2 add the following to site-file, HTTP (port 80) section

Header unset Strict-Transport-Security
Header always set Strict-Transport-Security "max-age=0;includeSubDomains"