This is my first time attempting to make a custom spell, and i know that wording is extremely important in DnD 5e, so i would appreciate anyone's help. 

This is what i have so far ... 6th Level Illusion Spell ... Range is 30 feet ... Concentration ... Called "Phantasmal Flames"

Pick A target that you can see within range. The target is enveloped in Purple Flames. The Target must make a Wisdom Saving Throw or take 6d6 Temporary Psychic Damage and is considered Stunned. If the Target saves on the throw they are sable to see past the illusion and the spell has no effect. 

• At the end of the targets turn the target can making make a Wisdom Saving Throw to attempt to break the Illusion, if they fail they remain Stunned.
• Once the Illusion is broken the target regains HP equal to the total amount of damage done by the spell.
• The spell can not kill a target, if the Targets HP is brought down to zero, they fall unconscious and considered stable. 
• If another creature attempts to help extinguish the target the Phantasmal Flames will spread to that creature as well. 

I would look at hold person and immolation as possible templates for wording. However, overall, it's a very unique spell and you may have difficulty finding other examples of the mechanics. I'm curious why it deals temporary damage and what happens if the target drops to 0 hit points during the spell. Would they regain the hit points despite being unconscious or dead?

I can't resist a mechanical challenge, so, using your bullet points, I came up with the following:

Purple flames envelope one creature you can see within range. The target must succeed on a Wisdom saving throw or it takes 6d6 psychic damage and is stunned. If the damage from the spell reduces the target's current hit points to 0, the target falls unconscious and is stable. At the end of each of its turns, the target repeats the saving throw. At the end of each of its turns, the target can make another Wisdom saving throw. On a success, the spell ends. When the spell ends, the target regains hit points equal to the psychic damage it took from this spell.

A creature can use its action to attempt to put out the flames on another creature. The creature must succeed on a Wisdom saving throw or be under the effect of phantasmal flames which lasts until the spell ends. 

"Phantasmal Flames
6th level Illusion
Casting Time: 1 Action
Range: 30 ft
Components: V,S,M (A small iron or steel symbol of a flame)
Duration: 1 Minute, Conc

A target you can see within range has to make a wisdom saving throw. On a failed save purple flames erupt around the target and the target takes 6d6 psychic
damage and is stunned for the duration of the spell. If this reduces the target to 0 hit points they fall unconscious and is stable."

I'm not sure how you want this spell to be of course but this is just merely for showcasing purposes. Tbh, 6d6 psychic damage for a 6th level spell isn't that much. 
While they are stunned, you gotta remember they get a save each turn. I don't know if they are meant to take the 6d6 damage each round or if their are just stunned.
But the wonderful "Astromancer" here seems to already have applied his 22 intelligence and help action so... this may be useless xD. Good luck though! ^^

well i was thinking that it would be a "on your turn as your action you can causes the flames to rage doing an additional 6d6 Psychic damage" would would them give them an additional opportunity to for the target to save from the spell.... 

in "Real World" context they get covered in flames and it hurts them .... but none of its real. once they figure out the flames arnt real, they realize they haven't been hurt at all.  It would act as a utility spell in regards it could be cast on a single enemy giving the party time to run, or as a damaging spell carving away health, however every time the caster decides to do more damage the target gets another opportunity to save .... making it a gamble ... and once the creature has saved, it regains the health making "knowing when to quit" an important part of the spell. 

Quote from Cporter5 >>

well i was thinking that it would be a "on your turn as your action you can causes the flames to rage doing an additional 6d6 Psychic damage" would would them give them an additional opportunity to for the target to save from the spell.... 

 An additional 6d6 psychic damage could quickly get out of hand, I would recommend 3d6. I would also take directly from immolation and say "At the end of each of its turns, the target repeats the saving throw. It takes 3d6 psychic damage on a failed save, and the spell ends on a successful one."

Quote from Cporter5 >>

in "Real World" context they get covered in flames and it hurts them .... but none of its real. once they figure out the flames arnt real, they realize they haven't been hurt at all. 

 With this in mind, I would change the saving throw to Intelligence and include the phrase "illusory purple flames".

I chose 6d6 damage because it seemed balanced. By making it a 6th level spell it requires  a level 11 caster ... at level 11 a rogues sneak attack damage is 6d6 which can be done every turn without limitation (assuming sneak attack rules apply) ... a level 11 caster gets 1x 6th level spell per long rest..

Version 2

"Phantasmal Flames
6th level Illusion
Casting Time: 1 Action
Range: 30 ft
Components: V,S,M (A small iron or steel symbol of a flame)
Duration: 1 Minute, Concentration

Illusory Purple flames envelope one creature you can see within range. The target must succeed on a Intelligence Saving Throw or it takes 6d6 psychic damage and is stunned. If the damage from the spell reduces the target's current hit points to 0, the target falls unconscious and is stable. At the end of each of its turns, the target takes 6d6 psychic damage and repeats the saving throw. On a success, the spell ends. When the spell ends, the target regains hit points equal to the psychic damage it took from this spell.

A creature can use its action to attempt to put out the flames on another creature. The creature must succeed on a Intelligence Saving Throw or be under the effect of phantasmal flames which lasts until the spell ends. 

It looks good except for "the target takes 6d6 psychic damage and repeats the saving throw." Spells deal damage after the reoccurring saving throw, not before.

Version 3

"Phantasmal Flames
6th level Illusion
Casting Time: 1 Action
Range: 30 ft
Components: V,S,M (A small iron or steel symbol of a flame)
Duration: 1 Minute, Concentration

Illusory Purple flames envelope one creature you can see within range. The target must succeed on a Intelligence Saving Throw or it takes 6d6 psychic damage and is stunned. If the damage from the spell reduces the target's current hit points to 0, the target falls unconscious and is stable. At the end of each of its turns, the target repeats the saving throw and takes an additional 6d6 psychic damage and remains stunned on a failure. On a success, the spell ends. When the spell ends, the target regains hit points equal to the psychic damage it took from this spell.

A creature can use its action to attempt to put out the flames on another creature. The creature must succeed on a Intelligence Saving Throw or be under the effect of phantasmal flames which lasts until the spell ends.