A few thoughts: (though you don't say what camera you're using - I'm assuming a DSLR, but if not some of the below might not work).
1) You can adjust the diopter on the side of the viewfinder and you can also get diopter adjusting eye attachments which can help if you wear glasses so that you don't have to wear them to look through the viewfinder.
I'm using 600D. And I always thought dioptre adjustment was only to make focus points sharper. But now I get it, as it is the power of the viewfinder lens, I can adjust in a way that the rays of light converge at my retina and not in front of it. Not a very practical option as I wouldn't want taking off my glasses for each shot and I hate contact lenses, It might help in certain situations though.
Time for an analogy of what a viewfinder is.
Consider how a view camera works. View cameras -- still used by some fine art photographers but more common in earlier days of photography -- were focused by projecting light onto a piece of frosted glass at the back of the camera. The camera does not actually have any film in it while it is being focused. It would be difficult to see the image on the frosted glass in order to focus the camera (especially if there's bright light shining on the back of the camera) so the photographer has a light-proof drape (typically a piece of black cloth) that they drape over the back of the camera and stick their head under it to focus in the dark. Once focused, they close the shutter, slide in the film hanger, remove the dark-slide protecting the film from light, and and take the shot.
Now imagine you're a myopic photographer and you aren't wearing your glasses. You'd have problems accurately focusing the camera because even when the camera is in crisp focus, the image projected on the frosted glass will appear to be out of focus to you.
Compare this to how a DSLR works... the camera is STILL projecting the image onto a piece of frosted glass -- which modern cameras refer to as the "focus screen". The pentaprism viewfinder effectively is the "black drape" so that when you put your eye to the viewfinder, light from behind you is effectively blocked so you can get a clear view of the image on the "frosted glass" inside.
This means that what you REALLY need to manually focus a camera are two things: (1) as one might imagine, the lens needs to be focused to project a crisp image onto the focus screen but (2) your EYES need to ALSO have accurate focus on that focus screen.
If the focus points etched on the screen are out of focus then what it really means is that your EYES are out of focus (since we know the etched lines on the focus screen should be crisp). This is an indicator that ANYTHING that is actually "crisp" on the focus screen will appear blurry to your eyes.
It is important that, whether wearing glasses or not, you adjust the diopter dial on the corner of your viewfinder to make sure those focus points appear to be nicely focused. It's easier to do this by pointing the camera at a nice plain white wall so that there are no distractions in the background and you can nicely focus the viewfinder diopter.
Now that you know the focus is accurate between the screen and your eye, you can trust that anything that appears in good focus on the screen is also correctly focused for the camera.