How do you get Umbreon in Pokemon HeartGold?

Answer 1

To get umbreon on Pokemon heartgold, you must max out an Eeevee's happiness at night. (8:00pm to 11:59pm) Some ways to do this are battling with it but DO NOT LET IT FAINT! Walk with it, and give it a soothe bell. That boosts happiness as well. To get a basic idea of how happy it is, take it to the house near the bike shop in Goldenrod City. Talk to the lady in the house and she will give you an estimate.

Level up an Eevee at max happiness during the nighttime.

you have to walk very very much and you have to train it lvl 25 or higher at night.

You don't have to level it up beyond any specific level. Eevee will evolve below level 25 as long as it has enough friendship.
first max out the happiness of the Eevee you get from bill by walking with it a lot. Then, at nighttime, train it to somewhere around lvl. 25.

Eevee can evolve at any level, even below lvl 25, as long as it has enough friendship.