There's a lot of different ways of doing this. I have two that I use. If I want to set up a system that can be lowered in case of emergency, I use an adjustable 3/4 Whoopie Sling with a Figure 8 and a carabiner that is designated for this purpose only. Meaning that I haven't used this for rigging. There are other ways but I had the supplies on hand. I will run the climbline through the figure 8 and hard lock it off. Then I add 2 half hitches, then a carabiner just for peace of mind. It's a little overdone but it doesn't cross my mind while climbing.
The other way is non lowerable. I'll get a wrap on the trunk of the tree then lock it off with a Bowline on a Bight with a Yosemite Tie Off. Then I'll get two half hitches then lock it off with a carabiner. Again, might be a little overdone but the extra 5 seconds it takes is worth the peace of mind.
Climbingarborist.com has a very good video on basal anchors.