I want to create my own homebrew versions of Aasimar and Tieflings and need some assistance with how to actually go about manifesting the designs in D&D Beyond.

As an example, for my Aasimar, I want to update the "Celestial Legacy" racial feature as follows:

At first level, you know either the guidance or the light cantrip.  Once you reach 3rd level, you can cast either the healing spirit or the lesser restoration spell once with this trait, and you regain the ability to do so when you finish a long rest.  Once you reach 5th level, you can cast the daylight or the fly spell once with this trait as a 3rd-level spell, and you regain the ability to do so when you finish a long rest. Charisma is your spellcasting ability for these spells.

I'm fairly certain that I need to use Options but I really have no idea how to make this all work.  Is there a functional tutorial that someone can recommend?

First question first: How much do you own on DDB? It might help if you can look at some pre-existing things.

For me, when I was homebrewing copies of the magic initiate feat, didn't use Options. Rather I just put the choices into the spells area. And under feats you could limit choices to a certain spell list or further to certain spells.

As for learning more about how to work it with creating your custom Aasimar, two races I'd recommend taking a look at (ie: homebrew copy them to see how they work) are High Elf and EE Fire Genasi. (Both are free to access.)
High Elf has a cantrip choice to make so that will give an intro to how it looks when there's many choices to make.
And Fire Genasi is one of the races with spells at a certain level.
So looking at those should help give you an idea on how to set it up.

Quote from VitusW >>

First question first: How much do you own on DDB? It might help if you can look at some pre-existing things.

For me, when I was homebrewing copies of the magic initiate feat, didn't use Options. Rather I just put the choices into the spells area. And under feats you could limit choices to a certain spell list or further to certain spells.

As for learning more about how to work it with creating your custom Aasimar, two races I'd recommend taking a look at (ie: homebrew copy them to see how they work) are High Elf and EE Fire Genasi. (Both are free to access.)
High Elf has a cantrip choice to make so that will give an intro to how it looks when there's many choices to make.
And Fire Genasi is one of the races with spells at a certain level.
So looking at those should help give you an idea on how to set it up.

I own a decent amount of content including both of the races you mentioned.  The problem is that what I'm attempting to do (i.e., give the player options for spells, instead of prescribed spells, as they level up) doesn't have a template in another race.

I have the option to fumble around with this but I'm trying to avoid the anatomy by the braille system approach if I can.

Use the Aasimar and Tiefling races themselves as the templates. Edit the traits that grant those spells, then expand the Spells subform. There you will see where the original spells have been added. Edit each one and add the alternate spell to the Spell field. So, for example, where it grants light for the Aasimar, simply add guidance to the same field where light has been entered so that both spells are listed in the field.

Do that for each alternate spell, then update the trait’s description and save the racial trait, then hit [SAVE CHANGES] for the whole race so the system picks up your edits. When someone selects that homebrewed version of the race, instead of automatically granting either spell, in the builder it will present a drop-down for each of those spell pairings where the player can select one of the spells of their choice. Does that make sense?

PS-I would have answered you hours ago, but I didn’t see your query. In future post homebrewing questions in the Homebrew forum and I’ll be more likely to see it sooner and answer your question faster.

Quote from IamSposta >>

Do that for each alternate spell, then update the trait’s description and save the racial trait, then hit [SAVE CHANGES] for the whole race so the system picks up your edits. When someone selects that homebrewed version of the race, instead of automatically granting either spell, in the builder it will present a drop-down for each of those spell pairings where the player can select one of the spells of their choice. Does that make sense?

Thanks.  I tried this and it works fine for cantrips (which get selected at 1st level) but it isn't working for the 3rd level features, which is disappointing.

It should work for all of them. When you say it isn’t working, what do you mean exactly?

I just tested it and it worked for me:

Yeah, I was not seeing that then I added the spells to the feature.  Even after I bumped up the test character's level to 3rd or 5th.

Please see the General Homebrew FAQ #s 6 & 6-B: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq). If that doesn’t sort you out post again and I’ll help you figure it out.