My goal is to install a new set of blinds in an existing window. It looks like a former owner tried to do the same thing and split the wood on the side of the window casing. Unfortunately the location of the split is exactly where I need to screw into. There are plenty of videos out there on how to repair split wood, but I can't figure out how to do it without removing the window. I'm not excited to go down that road - any other suggestions?
You don't have to repair the holes. Use longer screws that go all the way into the framing and hold the blinds' brackets solidly. For blinds that will be yanked and handled constantly that's the better thing to do anyway. After mounting the brackets you can decide whether to superficially fill and stain the holes. I think filling will look worse because a certain amount of damage such as yours is part of the vibe of an old house. Besides, it'll be hidden by the blind whether closed or open.
#6 screws should be thin enough to go through the existing holes without causing more damage. If you put a screw where there isn't already a hole, drill a hole 1/2 inch deep and big enough for the screw to pass through the sash stop (trim) without turning.
A #6x1-1/4" screw will pass through the sash stop (your damaged piece) and fasten to the jamb (the window frame) assuming that isn't also damaged. A #6x3" screw will pass through both of those, through any gap behind them and into the wall studs enough to make a solid hold.
Drive a couple of screws half way into the frame a little to the right of these holes. This provides a way to use a clamp. Inject good wood glue into the cracks, clamp it down (using the half-inserted screws) and let dry. Then for the holes you intend to use for the new brackets, drill them out to a consistent with where you can insert a dowel. Glue the dowel in, then predrill for your brackets. Finally fill the other holes and any remaining cracks with wood filler then stain. But you know what? It'll look bad, and even if you do all this you still ought to use longer screws that drive all the way into the framing .... so why bother?
Who says that piece of wood can't be moved?
That looks like a standard piece of window sash stop. Right next to the top hole is a hole from the nail that holds it on. If you look down the trim, you'll find several more nail holes. It would be easy to gently pry the sash stop off of the window frame to fix it. All you'd need is a stiff putty knife to slide under the edge and gently pry up right by the nails. Once you've got it started, you can use a larger tool to finish the prying.
Once you have the stop off, use wood glue injected into the crack (you'll have to pry the split apart to gently open the crack to ensure glue gets into the split, and doesn't just sit on top), then clamp it very tightly closed until the glue sets - probably 1-2 hours, check the instructions on your glue.
Once the split is repaired, you have options for dealing with the screw holes:
Of course, once you've got the trim off, you could go down to your local wood supplier (you may have to go to a lumber store or woodworker's supply store, this might not be a common trim piece to find at a big-box store) and pick up a new piece of window sash.
If you go this route, you'll have to stain it to match, and that can be a bit of an art as much as science. However, you'll have a brand new, solid piece of trim to hold the new screws.
Whichever method you choose, your best bet is to pre-drill pilot holes for the screws as you install the new blinds. That will go a long way to ensuring the wood doesn't split again.
Also, as noted in other answers/comments, use longer screws (and a deeper pilot hole) to get the screw mounted into the window frame and possibly the house frame and not just in the trim piece.
I would make sure the holes and the cracks are clean, then glue and clamp.
You might have to be creative with a clamp - one possibility is to have a large or long bar across the front of the window and a smaller one behind to pull the gap closed. Some long screws needed.
Use old hole.
You could put some of these in.
Then screw into them. You could use longer screws that go all the way to the bottom of the hole and into the wood there. You could do both of these things.
Or instead of a plastic anchor you could find a bunch of long slivers of wood that you have lying around (scraps, dowels, popsicle sticks, split pieces of firewood) and tap them them into the hole, maybe with some glue or maybe not if you have no glue. Then screw into that.
Fooling with the holes might force the split larger. Glue in the split and clamps for the split is a good idea. Some long thin screws perpendicular to the split and traversing it from the side next to the metal hardware will hold the split shut forever. Drill pilot holes for those long thin screws.
