How are you supposed to build space marines in 8th Ed?
TLDR, Have around 7 units for every 1000pts and give each unit at least 3 things that make them threatening

I'm writing this because I'm sick of waiting for someone else to answer this and I recently found A way to play Space Marines, though it's highly possible that it's not THE way to play Space Marines. Whatever the case, if this answer gets around 10 votes before someone else writes something and is right, then this will be my accepted answer.

Back to what my findings were, I believe that for every 1000pts, there should be 7 units in the formation and each should have at least 3 qualities or weapons given to them that make the opponent take notice of them. For example, you can give a scout squad shotguns, camo cloaks and a Landspeeder Storm. Instead of "just a unit of scouts" this unit becomes "a Toughness 5/Wound 7 model that moves 18" base, has fly, shoots up to 13 Strength 5 shots plus d6 Srength 4 shots on most turns and spawns up to 5 Scout Marines with a 2+ Save in cover when it dies". Attaining enough units that either have extended descriptions like this, or produce immediate reactions like a Landraider, creates a psychological advantage to your army I like to call The Actuality Effect. The Actuality Effect hinges on how effective your army is despite taking losses, such as a 10 man unit of Devastators loosing 5 units but still having its maximum output of heavy weapon shots. As for the effect itself, it is a mounting stress that occurs when the opponent is loosing units and is dealing seemingly ineffective damage to you. It's crucial to steadily build this tension in your opponent because, like in most strategy games, stress is the great obstacle your opponent must overcome. Stress makes you forget things, makes you focus on some things over other things, pushes you to chase or flee certain situations. Having enough units on the board, ergo unique places that DEMAND your opponents attention, will be enough to tip the odds in your favor.

Since building my lists this way, I haven't lost my last 3 games (2 vs Blood Angels, 1 of which heavily used Primaris units, 1 vs Orks). I won't guarentee that this works 100% effectively, one of my wins left me with 8 models on the board, but it performs drastically better than any of my previous lists this edition. Weather or not this is the best way to play Space Marines in general will be decided in time.