I once did a May ascent in a big snow year. We used the Smith Butte Sno Park and snowshoes. We should have used skis.
The approach is via the Aiken Lava Beds. Once you gain the lava beds, the navigation is pretty straightforward as you can actually see where you are going. The route goes over the top of South Butte. I suggest that you camp on or near South Butte and then go for the summit from there with an early start.
If you make good time, you can go out that day, otherwise you can spend another night in camp, which wouldn't be bad, as the views are great from South Butte.