For attributes:
Make an effect in effects.
Attach an enabler to the effect in effect_bonus_value_unit_set_unit_attribute_junctions.
Assign a unit set to the effect in unit_set_unit_attribute_junctions.
(Optional sometimes) Make a unit set in unit_sets and assign it to the unit(s) you want the effect to apply to in unit_set_to_unit_junctions.
Assign the new effect to a technology in technology_effects_junction.
(Optional) Remember to localize your effect so people know what it does.

For abilities:
Replace step 2 above with the same process in effect_bonus_value_unit_set_unit_ability_junctions.

Making custom abilities will obviously require additional work.