The CBB

Language family development if it’s a new language family
Phonology and orthography
Grammar
Words
Translate a children’s story or song into it
Try to translate more complex stuff like Wiki articles or pop songs with complex lyrics
Then if I don’t like it I think of the ideas I could’ve done better and make a new lang.
If I’m doing a language from a natural language family I do sound changes, make the orthography and phonology, the grammar, and the same process.

Depends for me. Usually, I start with the basic phonology and syllable structure, though sometimes I may tackle grammar or syntax first if I have a cool idea for either of those.

not 100% fixed.

but you should have concepts for 1. phonology, which includes the phonemes(what sounds do your languages have) and the phonotactics(how sounds are stacked together) and 2. grammar

besides, for words, you can start with function words like pronouns and something, and words on lists like the Swadesh list and the Leipzig-Jakarta list, and you may also think to add derivational affixes(like English un-, -y, -like, -ious, -ment, -tion, etc.) to help you extend the vocabulary in the future