Is there any tips to make a good one
Some "in general" stuff… well, for one, you'd need to make it realistic.
You mentioned that you're planning to include "Termination of witnesses". Why would the Foundation want/need to outright kill people who were involved? Why can't the Foundation have mind-wiped them with amnestics and let them go on with their lives? Usually if someone completely ordinary witnesses something anomalous happening, getting rid of them entirely is usually waaaaaay too excessive. The Foundation wants to make sure that the public don't know about anomalies, and don't know that the Foundation itself exists. If there are sudden disappearances or people ending up dead for no reason, the public will start to notice and get concerned.
Also, what is an "SCP Squad"? Are you thinking of something along the lines of a Mobile Task Force?