So altar of somekind it is. Here is something that is instructed in Dante abiels book; you would do a ritual with spirits of the dead where you would burn the altar after after Incantation that describes the altar being a portal for them. That would be done to a still portable yet big altar. Now, transform that ritual to fit your backyard(don´t worry, you don´t have to burn your backyard or accidently your house to do this). For example you could dig a hole, put some base inside of it and burn things in such a ritual. That is a safe way to do things(well, relatively). Not only that but you don´t need to make anything big to burn, you coiuld just use incense even. Here is the question though, how do you want to acquire shades?. The ritual to create a cemetery is easy but acquiring shades is another matter.