Hi
I think you need an extra 2 deer trophies if you've already used your other five.
Equip the deer trophies in one of your quick slots (1-8)
Go to the boss altar
When looking at the altar ... add the deer trophies to it by selecting the deer trophies in your quick slot.
The boss should then be summoned.

You will only spawn in your bed ... or back at the beginning if you have no bed.
You will need to build a small house/base in the forest with a bed ... select the bed as your spawn point ... and when you die you will spawn there.
You can have many bases throughout the lands.