If you are using OpenVPN and need a windows client then there is a nice client at https://openvpn.net/client-connect-vpn-for-windows/

However it is difficult to use the normal sample client.conf provided with OpenVPN and it’s better to have a client.ovpn file with the certificates all embedded

Easy RSA instructions for version 3

These are good easy to follow instructions: https://www.golinuxcloud.com/install-openvpn-server-easy-rsa-3-centos-7/

Sample client configuration files

Save these as a .ovpn

Client connecting to a server with username / password auth enabled

client
dev tun
proto udp
# remote server hostname or IP
remote remote.example.com 1194
resolv-retry infinite
nobind
ns-cert-type server
persist-key
persist-tun
# another example had these
# lines but OpenVPN Connect chokes
# on the [inline] option
;ca [inline]
;cert [inline]
;key [inline]
verb 3
keepalive 10 900
inactive 3600
comp-lzo
# if you have pam auth and don't have
# auth-user-pass-optional then you need
# to specify this to have the client
# prompt for user name and password
# this is for servers that both auth with cert
# username / password
auth-user-pass

<ca>
-----BEGIN CERTIFICATE-----
... 22 lines snipped
-----END CERTIFICATE-----
</ca>
<cert>
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 8 (0x8)
    Signature Algorithm: sha1WithRSAEncryption
        Issuer: C=AU, ST=NSW, L=Ringwood East, O=TGN-Ringwood East, OU=NSW, CN=remote.example.com/name=James McDonald/emailAddress=james@example.com
        Validity
            Not Before: May  4 13:35:33 2020 GMT
            Not After : May  2 13:35:33 2030 GMT
        Subject: C=AU, ST=NSW, L=Ringwood East, O=TGN, OU=Toggen, CN=tgn-lt01/name=James McDonald/emailAddress=james@toggen.com.au
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (1024 bit)
                Modulus:
                   ... 9 lines snipped
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Basic Constraints: 
                CA:FALSE
            Netscape Comment: 
                Easy-RSA Generated Certificate
            X509v3 Subject Key Identifier: 
                ...snippped
            X509v3 Authority Key Identifier: 
                keyid: ... snipped
                DirName:/C=AU/ST=NSW/L=Ringwood East/O=TGN-Ringwood East/OU=NSW/CN=remote.example.com/name=James McDonald/emailAddress=james@example.com
                serial: ... snipped

            X509v3 Extended Key Usage: 
                TLS Web Client Authentication
            X509v3 Key Usage: 
                Digital Signature
    Signature Algorithm: sha1WithRSAEncryption
         ... 8 lines snipped
-----BEGIN CERTIFICATE-----
... 23 lines snipped
-----END CERTIFICATE-----
</cert>
<key>
-----BEGIN PRIVATE KEY-----
... 14 lines snipped
-----END PRIVATE KEY-----
</key>

Client connecting with tls-auth

##############################################
# Sample client-side OpenVPN 2.0 config file #
# for connecting to multi-client server.     #
#                                            #
# This configuration can be used by multiple #
# clients, however each client should have   #
# its own cert and key files.                #
#                                            #
# On Windows, you might want to rename this  #
# file so it has a .ovpn extension           #
##############################################

# Specify that we are a client and that we
# will be pulling certain config file directives
# from the server.
client

# Use the same setting as you are using on
# the server.
# On most systems, the VPN will not function
# unless you partially or fully disable
# the firewall for the TUN/TAP interface.
;dev tap
dev tun

# Windows needs the TAP-Win32 adapter name
# from the Network Connections panel
# if you have more than one.  On XP SP2,
# you may need to disable the firewall
# for the TAP adapter.
;dev-node MyTap

# Are we connecting to a TCP or
# UDP server?  Use the same setting as
# on the server.
;proto tcp
proto udp

# The hostname/IP and port of the server.
# You can have multiple remote entries
# to load balance between the servers.
remote remote.example.com 1194
;remote my-server-2 1194

# Choose a random host from the remote
# list for load-balancing.  Otherwise
# try hosts in the order specified.
;remote-random

# Keep trying indefinitely to resolve the
# host name of the OpenVPN server.  Very useful
# on machines which are not permanently connected
# to the internet such as laptops.
resolv-retry infinite

# Most clients don't need to bind to
# a specific local port number.
nobind

# Downgrade privileges after initialization (non-Windows only)
;user nobody
;group nobody

# Try to preserve some state across restarts.
persist-key
persist-tun

# If you are connecting through an
# HTTP proxy to reach the actual OpenVPN
# server, put the proxy server/IP and
# port number here.  See the man page
# if your proxy server requires
# authentication.
;http-proxy-retry # retry on connection failures
;http-proxy [proxy server] [proxy port #]

# Wireless networks often produce a lot
# of duplicate packets.  Set this flag
# to silence duplicate packet warnings.
;mute-replay-warnings

# SSL/TLS parms.
# See the server config file for more
# description.  It's best to use
# a separate .crt/.key file pair
# for each client.  A single ca
# file can be used for all clients.
# ca ca.crt
<ca>
-----BEGIN CERTIFICATE-----
... 18 lines snipped
-----END CERTIFICATE-----
</ca>
# cert client.crt
<cert>
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            2e:81:27:87:8d:af:ce:70:d0:5d:12:03:bf:88:ff:cf
    Signature Algorithm: sha256WithRSAEncryption
        Issuer: CN=Easy-RSA CA
        Validity
            Not Before: May  5 02:32:36 2020 GMT
            Not After : Aug  8 02:32:36 2022 GMT
        Subject: CN=tgn-lt01
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (2048 bit)
                Modulus:
                    ... snipped
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Basic Constraints: 
                CA:FALSE
            X509v3 Subject Key Identifier: 
                ... snipped
            X509v3 Authority Key Identifier: 
                keyid: ...snipped
                DirName:/CN=Easy-RSA CA
                serial:...snipped
            X509v3 Extended Key Usage: 
                TLS Web Client Authentication
            X509v3 Key Usage: 
                Digital Signature
    Signature Algorithm: sha256WithRSAEncryption
         ... 15 lines snipped
-----BEGIN CERTIFICATE-----
... 18 lines snipped
-----END CERTIFICATE-----
</cert>
# key client.key
<key>
-----BEGIN PRIVATE KEY-----
... 26 lines snipped
-----END PRIVATE KEY-----
</key>

# Verify server certificate by checking that the
# certicate has the correct key usage set.
# This is an important precaution to protect against
# a potential attack discussed here:
#  http://openvpn.net/howto.html#mitm
#
# To use this feature, you will need to generate
# your server certificates with the keyUsage set to
#   digitalSignature, keyEncipherment
# and the extendedKeyUsage to
#   serverAuth
# EasyRSA can do this for you.
remote-cert-tls server
key-direction 1


# If a tls-auth key is used on the server
# then every client must also have the key.
#tls-auth ta.key 1
<tls-auth>
#
# 2048 bit OpenVPN static key
#
-----BEGIN OpenVPN Static key V1-----
... 16 lines snipped
-----END OpenVPN Static key V1-----
</tls-auth>

# Select a cryptographic cipher.
# If the cipher option is used on the server
# then you must also specify it here.
# Note that v2.4 client/server will automatically
# negotiate AES-256-GCM in TLS mode.
# See also the ncp-cipher option in the manpage
cipher AES-256-CBC

# Enable compression on the VPN link.
# Don't enable this unless it is also
# enabled in the server config file.
#comp-lzo

# Set log file verbosity.
verb 3

# Silence repeating messages
;mute 20