You need to create a global TCriticalSection and lock / unlock it when you access shared resources with code as shown below.
Create a separate TCriticialSection instance for each resource that can be used independently.
TCriticialSection is defined in unit SyncObjs.
uses
SyncObjs;
// one lock for each independent resource
var
myLock : TCriticalSection;
// early during application startup:
myLock := TCriticalSection.Create;
procedure TMyThread.Execute;
begin
//... lock it
myLock.Acquire;
// use the shared resource
// unlock it..
myLock.Release;
end;