I need help. I would like to create a magic item, but I do not know the best way forward. The following is what I would like it to be able to do, but outside of that, I do not know mechanics and balancing required, or even if what I am wanting is even possible.

Ranger Spellcasting Focus: A bracer that, without attunement, acts as a spellcasting focus for a ranger. This same item, with attunement, grants the wearer bonuses to stealth and perception as well as potentially a small boost to strength and/or constitution. I would also like for these additional properties to be unlocked as the wearer levels up in the Ranger class. 

Thanks in advance.

Bracer of the Hunter
Artifact
Attunement by a ranger

You can use this bracer as a spellcasting focus for your ranger spells.

Dormant. (uncommon - level 5) You have advantage on Perception checks and your Strength increases to 15 unless it is already higher than that.

Awakened. (rare - level 9) You gain the benefits of Dormant, plus you have advantage on Stealth checks and your Strength increases to 18 unless it is already higher than that.

Exalted (very rare - level 13) You gain the benefits of Dormant and Awakened, and your Constitution increases by +2 up to a maximum of 20.

Godlike (legendary - level 17) You gain the benefits of Dormant, Awakened, and Exalted, and your Strength increases to 21 unless it is already higher than that.

Thanks. What would this look like on D&D Beyond?

When Wotc designed how magic items work in this edition of D&D, they decided that magic item get created with a fixed/set amount of power imparted by their creators and that's it, they remain static for hen until destroyed or dispelled. So creating a magic item that scales with a PC in DDB's system is really not possible unless you set it up to specifically key off of stuff that PC already has that can/will improve, such as an ability modifier or proficiency bonus, but not really clas/character level as such. Some stuff can be finagled, but you will most likely need to create multiple versions of the item to represent it improving like you want since they aren't really sposta do that according to WotC's design concept for them, which is what DDB's system was built to emulate.

Agilemind‘s suggestion is great, but not quite what I had imagined when I was reading your OP. This is more what I had been imagining:

Woods-Person’s Wristguards

Legendary, (Requires Attunement by a Ranger)

[Insert obligatory flavor text here.]

While wearing these magic wristguards, you can use them as a spellcasting focus for your ranger spells.

While wearing and attuned to this magic item, you have the following benefits:

• You can cast the druidcraft spell as a ranger spell, using Wisdom as the spellcasting ability.
• You have a +1 bonus to your armor class when not wielding a shield.
• You have a +1 bonus to Dexterity (Stealth) and Wisdom (Perception) checks you make

If you have 7 or more levels in the ranger class, while wearing and attuned to this magic item you gain a +1 bonus to your Strength checks and saving throws, and attack and damage rolls you make using Strength. You also gain and an additional +1 bonus (+2 total) to your Dexterity (Stealth) and Wisdom (Perception) checks.

If you have at least 14 levels in the ranger class, while wearing and attuned to this magic item you gain a +1 bonus to your Constitution checks and saving throws, and a number of additional hit points equal to your ranger level. You also gain another additional +1 bonus (+3 total) to your Dexterity (Stealth) and Wisdom (Perception) checks.