Some new vocabulary in a sentence

このくつはかいてきです

I want to say "these shoes are comfortable" but I'm not sure how to make it plural, or if I need to make it plural at all.

First of all, I think we don't use plural forms in Japanese very often, particularly for things. Also, we usually consider a pair of something singular. A pair of glasses, a pair of scissors, a pair of pants are similar.

　a pair of shoes : くつ１足 or １足のくつ

　two pairs of shoes : くつ２足 or ２足のくつ

　three pairs of shoes : くつ３足 or ３足のくつ

　NOTE: Here 足（そく） which menas "foot" after numbers is a counter for shoes, socks, or something like that.

By "these shoes," if you meant ONE pair of shoes, your sentence was correct. if you meant SOME pairs of shoes, it would be これらのくつはかいてきです。 using これら (these), which is a plural form of これ (this), and we still don't make くつ plural.

I hope this helps.