Well, this is a tough one for me to answer, because for me fleshing out a lexicon is the most fun, maybe second only to creating a writing system. The only thing that’s not fun about is when I have a deadline. (Worst feeling when you’re doing translation and you don’t have the word you need. Any conlanger who’s done a relay knows that feeling.) No matter what your conlang is for, though, don’t sit down and create words because you have to if you can possibly avoid it. Any time you’re doing something because you have to it’ll be less fun.

There’s kind of a zen thing in creating lexical items, because you have to banish from your mind any thoughts you have about how many words you need to translate some text, or how many words your new project has compared to an old project, or someone else’s project, or a natural language, or compared to the Swadesh List, etc. Also don’t worry about whether the vocab you’re creating is useful or not. Clear all of that out of your mind. Instead focus just on the word you’re creating. If you can’t think of a word to create, think of a concept, activity, place or feeling. Start to think of all the vocabulary you need in English to describe that and see if anything sparks you. Then once you have your first word, let that word guide you to others. Think about expressions the new word might be used in, or other concepts it might apply to. Get as much mileage as you can out of each word. And then when this becomes tiresome, leave off. Come back again later and do it again with another concept. After a while you’ll notice you have a lot of vocabulary, including a bunch of words and concepts you may never have thought of otherwise. If you do this a few times it’ll become a habit, and then maybe vocabulary creation won’t be such a chore.

Honestly, though, more than anything else in any other artform, creating vocabulary can be really daunting and lead to existential despair. I mean, unless you have a writing project before you like A Dance to the Music of Time, creating a full vocabulary is a process that may never be completed in one’s lifetime, unlike most other artistic endeavors. The largest conlang vocabulary I know that didn’t borrow in massive swathes of words coined by millions of users is Talossan, which has about 30,000 or so words, but even that huge number is dwarfed by an average natlang’s vocabulary. You have to make peace with the fact that no matter what you do, the conlang is only ever going to get to a certain point, if you’re the only one working on it. You have to be content just doing a very little bit when it comes to the vocabulary—that, or create a minimalist language.