There are many ways to design a planner insert.
If you are really good with these office softwares, you could link your Word file with an Excel one and, using “Mail Merge”option, create pages with dates as well as Ray & Steve are doing for them templates.
Another way to create templates for calendar & co, it is use graphic programs. Them are a bit expensive respect a normal writing software, like the two cited above, but there are a couple of free applications made from Linux that can be installed in OSx and Microsoft too, e.g. Inkscape and GIMP, that could be a option if you would like to do something more design based with zero cost.
If you really want create complicate template … well you have to invest in a really nice Design program that give you the possibility to work with many layer, and move around in your drawing space friendly and with out freaking out searching for the tool that you need. There you have to look on your wallet and decide if this will be only an hobby in your life or if you really will need something like that in your future. Programs as Adobe Illustrator are incredible expensive and if you want to create only a couple of templates it is a cost that you may don’t want to effort, plus are not so easy to use. Recently I find this app named Affinity Designer that I am really loving. It is easy to use, cheaper (doesn’t involve any subscription feed) and have nothing less Adobe illustrator. If you are searching a good design software it is recommended app, moreover in my older mac work perfectly, meaning that it’s quite well developed.
And you? What are you using for creating your insert pages? Let me know in a comment down below.