without a spline ...
They will slide around a bit when trying to clamp them for gluing.
Tape them all along the edges on the outside with a painter's tape, then when you fold them along their edges they will stay tight together. A few brad nails will secure them while the glue sets.
OR you can use a long strip on the inside of the miter forming a ledge for the adjoining board's surface to seat, either one on each edge or two on 2 of the boards and none on the other two.
OR you can run a kerf in the center of the miter as suggested and use a strip of 1/8" ply or Masonite to locate the miters accurately. A table saw with a full kerf blade would do nicely. I would preglue the strips on one miter on each board, and let them set, rather than trying to work the strips in all at the same time while dripping with glue. Just don't let the glue oooze out onto the strips. That may be easier said than done...I donno? It won't take much glue in the kerf to work well.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)