Thankfully others have been here before me and have kindly posted their solutions online.
Firstly, there is this YouTube video that takes you through the basics. Just take note of the comments, and in particular if you get an error message when trying to run the commands shown in the video then do use these tips.
1. Run CMD as administrator (right click on it)
2. Type in:
set OPENSSL_CONF=C:\OpenSSL-Win32\bin\openssl.cfg (you may need to edit the path)Then press Enter on keyboard, then type in:
set RANDFILE=.rndAnd press Enter again. Then try the video instructions again and it should work.
To create the .p12 file, first download your iOS Distribution certificate file from the Apple Developer area, then you need to watch this video and use the file you just downloaded where it says developer_identity.cer - in my case the downloaded file was actually called ios_distribution.cer
To get the Push Certificate file (.pem) I had to source some different instructions. This time I found them courtesy of this page at BrightCove, under the section heading "Setting Up Push Notifications" - which is about 2/3 of the way down. Even those these instructions are for something else, they worked for me to generate a .pem certificate for the Tapatalk app, so I'm guessing they are fairly generic.
This should help those running Windows and looking to develop iOS applications without having to get a Mac.