Embalm is an activated ability you can activate if the creature card with embalm is in your graveyard. Notably you’re not casting the card from the graveyard, so things that counter spells won’t work against the embalm ability. The original creature spell can be countered, of course, but that just puts the card in the graveyard.
To activate an embalm ability, simply pay the embalm cost and exile the card. You do this any time you could cast a sorcery, meaning during your main phase when nothing else is happening. When the ability resolves, you create a token copy of the card, but as you’d expect, going through the process of mummification causes some changes. It’s got that whole undead thing going on, so it’s a Zombie in addition to whatever creature types it used to have.