EzekielRaiden
Follower of the Way
Being perfectly honest: I wouldn't. I genuinely do not think 5e is capable of supporting it with only its existing mechanics. The places where it would need to be open-ended it is instead incredibly restrictive, and the places where it would need to be more restrictive (I would personally say "consistent") it is instead so open-ended it borders on empty. Indeed, I'm not even sure it is possible without actually rewriting the rules (e.g. as Level Up has done), that is, I'm almost convinced (call it 60% convinced) that even a new class wouldn't cut it.How would you build a Warden, using only existing 5E rules and components?
I feel the same way about the Warlord, for example. Shaman and Avenger are borderline cases: if you're really rigorous about restricting things to absolute 5e RAW then I don't think they are doable either, but if you allow some small tweaks I think they can work just fine. Namely, for Avenger, let the Zealot Barbarian apply Barbarian features to Dexterity stuff and use Dexterity to hit/damage with big weapons (perhaps an "Avenger Training" feat to justify such things and make an Oath of Enmity option for Rage usage?) That would make a solid Avenger-alike in 5e terms. Likewise, some relatively minor tweaks to the Wildfire Druid would absolutely bring it at least within spitting distance of the Shaman, though I'm not entirely sure how to bridge that lingering gap between "still a bit disappointing" and "an acceptable translation even if it had to change."
Warlord and Warden, on the other hand, I don't actually believe can be built in 5e. You can make things clearly trying to imitate them. But they don't actually make it to "acceptable translation" territory the way "fully Dex-based Zealot Barbarian" would. The kludge of "play Ancients Paladin with a two level Archery Blade Warlock dip" shows how difficult this is: you can't get your core class concept until at least level 6, and that only if you plag variant human AND get your DM to approve an Unearthed Arcana fighting style that never made it to print. All of that, just to kinda-sorta half-resemble a Warden that doesn't have any forms.
5e has the clear design goal (one of the few explicit and identifiable ones therein!) that you should have your basic kit by level 2 or 3. Making it so the "Warden" wouldn't have it until 6th level and that only if you play a specific race is pretty far off that mark. Note: your suggestion cannot be completed as described, because if you multiclass Warlock at character level 4, you don't get a feat at that level. You must be Paladin 4 or Warlock 4 to get your first feat. And for anyone playing anything other than variant human, hope you're happy not getting to play your concept until 10th level!