I am trying to create a subrace of elves for use in my campaign. 

Have all the normal elf traits, + 2 Dex, Fey ancestry, Trance etc, 

Then essentially adding 2 things. 

Ability score increase (to be chosen in character creation), and a per long rest ability. 

Struggling with the homebrew builder, and was hoping for some assistance?

Add Khan elves.
Choose one of these subraces,
In some worlds, these subraces are divided still further (such as the sun elves and moon elves of the Forgotten Realms), so if you wish, you can choose a narrower subrace.

Ability Score Increase. Your Dexterity increases by an additional 1 or your Constitution score increases by 1.

Khanic Weapon Training. You are proficient with the javelin, spear, short bow, and longbow.

Power from Pain. When you hit a creature with a melee or ranged weapon attack, you can use your bonus action to gain power from pain. You suffer (1d4 + your character level) psychic damage

The creature suffers an additional (1d4 + your character level) psychic damage at the end of its turns for 3 rounds.
The damage suffered /dealt increases to
1d6 + Your character level  /3d6 + Your character level at level 6,
1d8 + Your character level /3d8 + Your character level at level 11,
1d10 + Your character level /3d10 + Your character level at level 16,
After you use Power from Pain, you can’t use it again until you complete a long rest.

Languages. You can speak, read, and write Auran.

Is there something in specific you're having trouble with? Best bet is to copy an existing race or subrace. I'd suggest using the High Elf as a template, that'll give you a form of weapon training and an ASI to adjust. Be sure to save it as soon as you've created it before you make any changes.

Thanks for the reply 

Using the High elf as a starting point is certainly an improvement. 

My Subrace has an either or in terms of ASI, how does this work?

Also I feel my per rest ability a little complex to add in?

New (again) to forums forgot how this stuff works. 

Think iIve made some progress from your advice. 

The big one is Power from Pain - feels vastly complex too add to the race.

Power from pain is different from how most abilities in 5e work. Additional damage based on level, self harming, no concentration, no immediate payoff. And thematically, how does it even work? Why is the ability what it is?

Also being able to get +3 to a stat from race is pretty strong. (On that note there is no way to pick out which abilities they can choose from, just have to hope they follow directions.)

I would make Power from Pain a homebrew spell that's modeled on Vampiric Touch, and add it as a racial feature that way, which will also let you set it to use once per day, I'm pretty sure. As for the ASI, you can do Modifier type: bonus; Modifier subtype: 'Choose Dexterity or Constitution'; and a fixed value of 1.