Let's say you have an SSH private key. For example, on a Linux system, your user's /home/$(whoami)/.ssh directory may contain something like this.

~]$ ls -lisa /home/$(whoami)/.ssh
total 16
 1091637 0 drwxr-xr-x. 2 john.doe admins   79 Dec  4 10:50 .
16919760 0 drwx------. 3 john.doe admins  111 Dec  4 10:50 ..
 2966257 4 -rw-------. 1 john.doe admins  734 Dec  4 10:50 authorized_keys
 2966249 4 -rw-------. 1 john.doe admins 3414 Dec  4 10:13 id_rsa
 2966255 4 -rw-r--r--. 1 john.doe admins  763 Dec  4 10:13 id_rsa.pub

In this example, the id_rsa file is the SSH private key. The private key file should contain something like this.

-----BEGIN RSA PRIVATE KEY-----
MIIEpAIBAAKCAQEAl/qBJwMtUFnLns9cGCjnC1EKuWwwK+ItcLF7WoDOISxfmE/l
1TSBEos16nyqv6qMQOfNu/1sGJ8IHCWeqMuWJO4nY3RebOJ2jinw2aiLtMkzQjRQ
KGVs62h1VzLx2n8IVLXOSnEmTRRRpNIH5vWx2EEBtchtCXGiUcJIRt8Q1UVsfTbp
H7GvAVDqGMfnXqizqbGtfmDZTtAmq4JYReUUej1NnE8O8hLFNXqcrioo27puF8Ti
gPoTpp9p8ncidktd3NWck9jJGvEoaA3oxsKMgZGlPviMkhTv70Ovh7ShPpZ38ETB
7SkOcn6W4hbDHFqSihmrf0S7JaSN/RYKY2wVMQIDAQABAoIBADFOVkRf5X2iBEub
npc//eVP/5i0ntQ95twe2JQ+IaHlkYYKtdf71rw7adyPYZ4ZKLOOANMynrVGMw1e
-----END RSA PRIVATE KEY-----

Let's create a PEM file that contains the contents of the private key file.

cp id_rsa id_rsa.pem

Here is how you can create a PPK file using the PEM file.

Windows

1. Download and install PuTTYgen from http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html.
2. Start puttygen.
3. Ensure RSA is selected (since the PEM file contains an RSA private key in this example).
4. Select Load and select the PEM file.
5. Select Save private key button to create a PPK file.

Linux

On a Debian distribution (Ubuntu, Mint), apt-get can be used to install the putty-tool package.

apt-get install putty-tools

On a Red Hat distribution (CentOS, Fedora, Red Hat), dnf or yum can be used to install the putty package.

dnf install putty

And here is how you would create a PPK file using the PEM file.

puttygen id_rsa.pem -O private -o id_rsa.ppk

Add the content of the id_rsa.pub file to your users authorized_keys file.

~]$ cat /home/john.doe/.ssh/authorized_keys
ssh-rsa AAAAB123456. . . asdfasdfa213123w== john.doe