Things you need to know :
-you need a forum account to report a player.
-reasons for reporting are (Cheating,abuseany kind of abuse : harassing, blocking ,team-killing,being racist,spamming, using bugs... )
-you can report by screenshots or demos using forum links
lets jump to the topic :
1/First you need to click on the strike list see the image

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2/You need to log-in with ur account [Follow the red numbers

(if you dont have a forum account then u need to create an account by pressing register see the green circle )

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3/Press make a new report

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4/New page will show and u need to type :

2==>Steam id or valve id (to get this you are supposed to be on the server then u use ur console and type the command "status" copy past the id of the player you want to report there)
3==>Name of the player
4==>pick the server where that player was playing
5==>Why do u want to report this player (pick a reason and make sure u picked the right reason)
6==>if u have anything to say you can type there for exemple (demo time when did he abuse / translations if he is harassing with a non english language...)
7==>you need to take a screenshot of the id that ur reporting if u have it then all u have to do is to drag that image and put it on the box there
8==>if u have a demo then u drag it and put it there just like 7 (to find ur demos you just need to go to ur cstrike folder and u find it there with the name that u have chosen (remember demos are the onces ending with (***.dem)
HOW TO RECORD A DEMO?
if u want to record for exemple cheaters you open ur console and type "record cheater1" or any other name "record noob" once u get ur proofs (speedhack,aimbot...) you open ur console again and type "stop".
9==>if u have any screenshots you think will help accepting ur report just drag them there (harassing, team killing...)
Things to remember :
-Dont spam the reports
-you cannot report someone just by his name or putting into the report non sense demos or screenshots ,you need to make sure you have solid proofs before you report
-if you report a player it will take some time for ur report to get accepted or declined depending on ur proofs.
-if you saw someone has already reported the player that u want to report then u can just reply in that report by putting ur proofs as a comment (no need to make a new report).
Hope this topic helped you =)