"Ottar made me
a temple of stone,
and the stones of that temple
glisten like glass,
reddened with fresh blood
from sacrificed oxen -
Ottar believed faithfully in the goddesses"
-- Voluspa en skamma, Translated by Jackson Crawford
While altars and shrines are commonplace in modern pagan practices, we don't have a ton to go off of when reconstructing/reviving old Norse practices. Our understanding of hörgar (a form of Old Norse shrines) comes primarily from three sources within the Poetic Edda:
In Völuspá, the Aesir gods meet at Idavallen and construct hörgar (shrines) and hofs (temples).
In Vafþrúðnismál, Oden (in disguise) references Njord ruling over many hofs and hörgar.
Finally, in Voluspa en skamma, Freja references her follower (Ottar) building a hörgr for her. This account gives us the best picture of what a hörgr is.
Freja describes the hörgr as a shrine made by piling stones. These stones are then ritualistically reddened with sacrificial blood as an offering to Freja.
"Sacrificial blood of oxen" isn't a common offering to Norse deities and spirits anymore, but building shrines can still be a useful way to anchor deities, ancestors, etc. to our spaces. What are ways we can bring the practice of building a hörgr into our modern practice?
To start, you can collect stones for a hörgr. Collecting the rocks can be a devotional act in itself. In pagan and witch circles, we often talk about asking permission before foraging (rocks, plants, etc.). However, you should also be mindful of where you are collecting stones from -- moving rocks can be detrimental to an area's ecology. Avoid stacking rocks or moving rocks away from public land, wildlife preservations, etc. Animism is important to Norse paganism, and part of acknowledging our interconnectedness also means respecting the life of others over our own desire to have an aesthetic practice. Minimize your impact!
A hörgr can be build inside or outside, depending on your needs as a practitioner (again, just consider the ecological impacts on where you build). Stacking the rocks can be a meditative process -- take your time to find an arrangement that works for you. Similar to modern shrines and altars, many Norse pagans choose to decorate the rocks and the surrounding area with images/symbols associated with the deity/spirit(s) the hörgr is dedicated to. Get creative!
Dedicate your hörgr to the intended spirit/deity with an offering and some words. Present offerings as often as you see fit! As a substitution for blood, I like to pour alcohol/water over the rocks (again, if you've built your hörgr outside, think about the environmental impact of your offerings). Your hörgr can also be used as a place to meditate and commune with deities/spirits.