2020, I have replaced many focus screen starting with the D40, D700, D7100, and the D3200. I don't think the D3300 would be any different. All are super simple to do.
The D3200 is a simple clip that a mini flat head screwdriver will pop upwards.
What I do is a little more in depth.
I vacuum the black box before I do anything to remove the focus screen, I do this to vacuum out any stray fibers and dust. I use the attachment hose without and fitting. The hose is just a hair smaller than the metal lens mount. DO NOT seal the hose but rather leave a 1/4 inch gap between the hose and lens mount.
2) I pop the metal retaining clip that holds the focus screen, I use a mini screwdriver to do this.
3) Turn camera over and usually the focus screen will lean out, if not a soft slap to the camera body should free it.
4) remove the focus screen, there is a brass frame and a black paper frame under the focus screen (Between the focus screen and focus point screen) yes there is actually two screens. Sometimes these two frames will lean out too, I remove then as well as I also remove the focus point screen. I'll get to that in a few steps.
5) with focus screen out hold the camera upside down and look through the viewfinder, your looking for dust, tiny hairs and debris on the focus point screen. usually there is debris that gets in between the two focus screens. I use a very slightly "Very Slightly" damp Q-Tip and clean the focus point screen until it is clean when looking through the viewfinder. Be gentle as it scratches/mars easy and be patient as it takes some time. There will most likely be at least one stubborn speck, and this is why I remove both. When both are removed the pentamirror is also easy to clean.
6) After I remove the two focus and point screens and the two frames, I re-vacuum the black box to vacuum out any dust in the pentamirror being the pentamirror is now opened with the two screens removed.
7) I take the focus point screen and rinse it with water and let it dry.
8) After the focus point screen is dry I reinstall it. I don't bother with rubbers for my fingers because those are really not needed, just pinch the very edge of the screens. Recheck for debris, usually the focus point screen goes back in clean.
9) with tweezers and a soft touch install the black paper frame. look through viewfinder again to check for debris. Re-clean if needed as tiny debris will be an issue and is just part of the deal.
10) Once the black paper screen is in on top of the focus point screen and the viewfinder is clean/clean enough for your taste, with the tweezers now install the brass frame, again looking through the view finder for any debris.
11) Once happy with the lack of debris when looking through the viewfinder now place the focus screen in. Look through the viewfinder and repete cleaning until your happy. It wont be a one shot so get ready to remove clean and remove and clean. The tiny black fibers of the mirror slap pad really get all over the place so be patient.
12) Once you are happy with the view when looking through viewfinder, now use your index finger and swing down the metal retaining clip and press it under the tab lock.
Your done.
As far as people saying get a new camera or your camera is old is BS to be honest. My D3200 gets used just as much as my D7100 because the image sensor is much better in way of dead pixels (Hot pixels) the D7100 has more dead pixels. My vintage D200 still gets used just as much as my D7100, in fact it took 16 different D200's over the years to get a good one. This D200 I don't have to worry about editing dead pixels at all. And that's rare for a CCD (Charged Coupled Device) I rarely use my D7100 it has a junk imager lots of dead pixels, all my cameras were bought USA and new. My vintage D700 blows away my D610. Why, because the D700 has way less dead pixels than my D610.
What I am trying to say is every camera's imager has flaws, if you have a camera with a great imager why risk getting another camera that might not have such as good imager in a dead/hot pixel level. I am a photographer, soccer moms/dads are the type of people who must have the newest to look good in public whereas a real photographer just needs any camera. Looking good or spending money don't always yield the best image.