These shrines were the pinnacle and most common in Ancient Roman Polytheistic Household Worship or Religio Romana. They were used for the purpose of worship and honouring the gods that were named after, the Lares.
The Lares were a diverse group of guardian spirits commonly as spirits of the crossroads, Lares Compitales and had a respected festival called Compitalia where people offer and worship the Lares at their crossroad shrines. Lares of the household were called Lares Familiares, ancestral guardian spirits of the home, property, household, protection, and providers of prosperity and abundance when pleased. They were depicted as dancing youth carrying libation dishes and cornucopias sometimes winged and sometimes not.
Historical mosaic paintings showed the Genius Loci but it’s not what modern paganism or witches think it is. The Genius Loci is linked with the patriarch of the household so the man of the house essentially, if the Genius Loci is pleased the man who was the main provider at the time would be successful and stable and secure within his family and career. The Genius Loci was depicted as a snake and the patriarch would also be painted on the wall as well between the dancing Lares but not always. Sometimes it’s a household god or patron god of the family. Lares would have statues as well (angel statues are a great substitute btw). The gods could be Vesta, Bacchus, Janus, or the Penates. You have the Lararium shrine then the altar space for rituals and offerings to take place.
How you create and display your shrine is going to be up to you. Some will go reconstruct historical ones that look like mini temples and in the center the mosiac painting or actual small temples to house the idols and offerings. There is nothing wrong of having a blank wall shelf and a printed out picture of a Lar or statue which again Angel statues are great examples as along the Lar is included and enough room for rituals to take place you’re all set. Jewelry cabinets are great and empty Catholic wall shrines are also great. Lararia are heavily personalized you can go expensive to inexpensive as much you want build it up as you go.
Again heavily personalised and private no Lararium is going to be the same. Usually it was place in the family room or living room but there is no rule saying you can’t have one place in your bedroom or closet as along there is enough room to be comfortable making offerings and prayers is no big deal. They are worshipped alongside household deities commonly with Janus and Vesta threshold deities, but again any deity can be worshipped alongside them ideally household deities but your patron deity is considered a household deity. Hekate or Hermes and Apollo are great examples. You can make petitions to the gods at your shrine, Lares traditionally are witness to big family events such as weddings, adoptions, birthdays, graduations, etc. also Lares have a seat at the table so dummy dinners where you bring their statue to the dinner table or bring them a plate of food, it’s also believed that when a piece of food drops on the floor it belongs to the Lares.
Now if you’re familiar with offering to deities especially to Hellenic and Roman deities it’s really about the same way. Libations of wine, water, juice, etc flowers, incense, fruit, pastries, and candles, it’s said that when someone brings in their harvest of crops, fruits, and produce it is offered to the Lares and Penates first. Modern day can be seen as you get a bag of apples, when you open the bag for the first time take the first apple and clean it and offer it to the Lar of your household, plain and simple. Of course it can adapted but just an example.
Lares are part of your ancestry and family so feel free to talk to them like family and see them as family. Pray for good health for not just you but your family members, financial stability, abundance, cleansing of the house, household protection, etc.
DISCLAIMER: You don’t need to have Italian ancestry to worship or connect with the Lares or have an Lararium, these shrines have been found outside of the Rome like in Gaul and Britain, the Romans loved sharing their religious practices. But it is a strict Roman tradition not a Hellenic tradition you can be a Hellenic polytheist and have an Lararium but don’t label it as Hellenic polytheistic and do not remove the Lar from the Lararium. It’s attached to the shrine within itself.