I read in a guide that you should type -tools when TF2 starts and do something from there. Is this not necessary?
you do this to find the name of the particle effect you want. If I recall correctly you should do it like this:
First use GCFScrape to extract all particle files at tf/particles.
Now start the game in -tools mode and select the particle editor mode at the top. At the topleft somewhere you can select to open a particle file. you open the ones you have extracted and look through them. Every particle file includes multiple particle systems, you can play a preview of all of them. Copy the name of the particle effect you want to use and paste it at the Particle System Name of your info_particle_system entity.
It probably helps looking a bit through this video:
https://www.youtube.com/watch?v=70GhO6z5aKM to get the idea how to open the particle files and names in -tools mode.
Don't forget to delete your extracted particles (or move them to a custom folder like tf/cake) to prevent your particle systems being out of date in case Valve updates them.