To change how the kill feed looks, there's a loader program that adds some functionality to CS called HLAE (Half-Life Advanced Effects) from
https://www.advancedfx.org/. You can skip these bits if you just want the plain video of the game. Unzip the program, run HLAE.exe, go to Tools --> Developer --> Custom Loader. You should get a screen like this -
For the program path bit, browse to the CSGO .exe file and select it. For the DLLs bit, browse to the HLAE folder and select the file AfxHookSource.dll. Leave the command line arguments as is especially -insecure (stops the game from connecting to Steam servers) and -window (so you can use a second monitor without having to alt+tab out of the game). Should look like this when done -
Then click OK and the app will launch CSGO for you.
Find the demo you want to use and then type this without quotes into the console once the demo is playing -
"mirv_deathmsg debug 1"
That displays info in the console about the kills happening in the game...wait till the player you're wanting to video gets a kill and then look in the console for the entry. Take down the value (it will be a number) set for the uidAttaker variable for that kill - that's the id for the player. Let's say it was 25 so the stuff below makes sense.
Then enter these into the console without the quotes -
"mirv_deathmsg block !25 *" ...this blocks all kill feed notices for players other than the one selected.
"mirv_deathmsg cfg lifetime 45" ...sets how long the messages stay on screen in seconds. Set it to however long you need.
"mirv_deathmsg highlightid 25" ...draws the red block around the kill feed message.
"cl_draw_only_deathnotices 1"...removes the HUD except for the kill feed (no radar, health bar, that panel that demos have in the middle with stats)
So now we can record. I changed all the games settings to high before and resolution to 1080p (if you have a 1440p monitor, it might be good to use that) to make it look better. Then I tried OBS first to record but that dropped the frame rate in game really low - looked very stuttery. The recorded in Nvidia's Geforce Experience records off the video card's frame buffer (or seems to) because the game is unaffected when I used that to record. Also very easy. Install Geforce Experience and make sure the in-game overlay is enabled. Change the recording settings to 1080p 60fps (I also had to turn the mic volume all the way down or there was a hiss in the video).
Hit ALT+F9 to start recording and the same again to stop. That should do it....by default, it puts videos into the My Documents folder - you can change that.
Then uploading to Youtube. The first time I just uploaded it as is but that got compressed and looked crap. After reading a bit, seems 1440p gets VP9 encoding (better) while lower resolution gets AVC1 (not so good). I used Sony Vegas Pro to upscale the video to 1440p and set the render settings like so -
There are lots of effects and stuff in Sony Vegas Pro (I used Adobe Premiere before) but I haven't used them before - maybe next time. Once it's done rendering, you can upload to Youtube and hope they don't decide to squish it into mush for mobile devices.