Once you get the blade set at the correct angle use the clamp that came with your saw to hold the board in position as you cut it.

 

Those are some very complex miters and bevels. Some people think miter saws are precision saws and they are not. They are are accurate - to a point. even the best ones. However with any saw an angle can be dialed in with repeated test cuts using incremental adjustments, AKA "sneak up on it".

I suggest you get a good digital gauge. Igaging makes a good one. Dp not buy a General they are horribly inaccurate.

Always start with material cut a little long to allow for the "sneak up on it".

 

Is the bearing on the blade tight, bearing at pivot point? Arm travel vertical? Or, can you wiggle the blade using your wrist on the handle and make it deviate to left and right.

It can be tough to get a perfect setup depending on the quality of your equipment you're adjusting and the precision of the tool your using to set the equipment up.

Scrap lumber is always a great "tool" to have laying around to give test cuts anytime you set up for an operation. Many times, I end up with an entire project made out of scrap-test pieces! (Of course you can see the progress partial cuts in the joints from where the saw was "just a little bit off" before I correct it...)

 

Because there are 5 intersecting bevels any error become amplified by 5 times!
If you didn't compare them before you assembeled them you won't know which pieces were too long or too short.
If the bevel angle was off by even 1/2 a degree, the joints wont mate properly.
A stop on your miter saw fence will assure accurate length.
A digital bevel gage would assure you have the proper bevel angle.

There's two ways to cut those bevels:
1. Angle the blade or to 90 degrees minus the angle for the setting and then use the digital gauge to verify it, not the graduations on the saw!
This would means you hold the piece flate on the saw table.

2. Hold the piece vertically on the fence and set the saw/motor assembly to 90 degrees minus the angle, like you would a miter.
This may be more precise, since there's a finer, more accurate setting than on the tilt mechanism.

 

My spouse and I were testing angles for a shelf we wanted to tilt back ever so slightly. I was shocked by what I learned. Even 1/10th of a degree made a difference, and 1/2 a degree was "huge".

She buys picture frames at the thrift store and has me cut them down on the miter saw, sized to match her art work, and sometimes I make picture frames. You may think that it is easy to get perfect 45 degree angles on simple picture frame pieces. It isn't. It took me a long time to understand what it takes to set up a perfect 45 degree cut on the miter saw. Especially because my spouse has a basic cheap Craftsman miter saw. Setting it up to make precise, repeatable cuts takes time. Digital tools help.

(Actually, I set it up for 44.95 degrees, just the thinnest hair under 45 degrees. The tips touch and flex together ever so slightly as you clamp and glue them. It works for me; it was a trick I learned from a gnarly old guy in our woodworking club.)

I saw your star and thought, "Well, the angles line up pretty good. I wonder whether I could do better. I doubt it."

Consider using a table saw with a sled and/or jigs instead. You can achieve a very high precision with it for miter cuts. I know a bunch of people who turn segmented bowls. They need to cut precise angles that add up to a 360 degree circle like your star. They all use jigs. Nearly all of them use table saws.

Precise alignment of your saw is also a necessity, whether you are using a miter saw or a table saw.

 

An easy test for your 45 degree angles is to dry fit them together forming a 90 degree angle, hopefully.
Cutting the angles at less than 45 is also the recommended way to get joints that close up.
I've posted this a few times here;

 

Cutting the angles at less than 45 is also the recommended way to get joints that close up?
Seems to me that is a way of creating a gap for a poor glue joint, works in construction moldings, but never thought of it as a best practice in fine woodworking.

 

The trick has worked for me with picture frames. It fixed my issues with gaps and inconsistency with previous picture frame miter cuts. The joints close up. If there is a gap, it is invisible to my eye.

Another way to think of it is: "Strive for a perfect 45 degrees, but ensure that any error falls on the "44" degree side of the angle."

 

The trick has worked for me with picture frames. It fixed my issues with gaps and inconsistency with previous picture frame miter cuts. The joints close up. If there is a gap, it is invisible to my eye.

Another way to think of it is: "Strive for a perfect 45 degrees, but ensure that any error falls on the "44" degree side of the angle."

Of course, but do strive for 45 degrees, depends on the accuracy of the saw to be consistent as well.

 

Another way to think of it is: "Strive for a perfect 45 degrees, but ensure that any error falls on the "44" degree side of the angle."

YUP, I'd much rather have a closed miter joint than one with a gap, even if it's a "hack" to get it that way.
I don't give a bleep how it was accomplished, just that it looks "perfect".
If we are talking in 10ths of a degree no miter saw I know has that kind of indexing nomenclature.
A digital angle gauge is the only means I know to get that kind of accuracy.

 

For mitering larger pieces I prefer a sled on a table saw. That said, a few things you can check. If your miter saw is of decent quality there should be a way to go through a setup procedure. They all claim to be set up out of the box, but my experience has shown that not to always be the case. Make sure the blade is running at exactly 90 deg. to the fence when set at 0. Make sure the blade is a perfect 90 deg. perpendicular to the bed and that when set at the 45 deg. indent that is accurate as well. Do the same to the bevel if your saw has one. Second, use a quality blade and try not to use a thin kerf blade to avoid deflection. If you must use a thin kerf make sure your blade is sharp and if you can use a blade stabilizer. The advice above regarding cutting a 1/2 degree or so under what the miter calls for is sound. I would also suggest splining the miters or using loose tenons to aid in assembly, clamping, and alignment. If splined or tenoned you should not need to nail the miter.

 

The advice above regarding cutting a 1/2 degree or so under what the miter calls for is sound.

1/2 degree (0.5 degrees) is way too much.

Speaking for myself, I use 1/2 of 1/10 = 1/20th of a degree. As I said in my post, "44.95 degrees", which is 0.05 degree off, a nearly imperceptible amount.

 

I don't do many picture frames but I know they are the most difficult, especially when there's a contour or compound angle.

I've been intrigued by Patrick Sullivan's miter sled. Seems like it would be super accurate.

I have also used a shooting board with an adjustable fence. The one on the left is just a 45 with a few degrees of adjustment both ways. I can't find a pic of the fully adjustable one.

One issue is tweaking angles, you make one leg shorter & you can end up chasing your tail.