There are books that teach you how to write Devanagari, for example,
Rupert Snell: Read and Write Hindi Script. Teach Yourself. McGraw-Hill, 2010.
Elvira Friedrich: Einführung in die indischen Schriften. Teil 1: Devanāgari. Third edition. Hamburg: Helmut Buske, 2012.
You can also find many YouTube videos that teach you how to write Devanagari, such as ...
There are three key things to pay attention to (I've found):
place of articulation (~ tongue location)
aspiration (~ whether air comes out while you speak)
voicing (~ whether vocalised - think sneezing 'choo vs. AAH-CHOOOO)
Wikipedia has a convenient table:
of the top row, I think (in my beginners' experience) only nasalisation needs conscious thought, ...
The stroke order is not important. You can draw the "ka" as you wish. But most people follow the way that you described in the first bullet as it's more natural that way. Usually people draw the horizontal line at the end as is correctly explained in your question. But all these are just for the sake of convenience. As the stroke order is not important to ...
Devanagari letter has vowels, Consonants and Compound letters.
alpaprāṇa(very little air flow through mouth) and mahāprāṇa(more air flow through mouth) are types of Consonants which are sounds similar for new learner. ex: Ka(क) and kha(ख)
Compound alphabets look similar for new users, ex: letter ka(क) is compound with e becomes ke(कॆ).