Buy ETS like HT weapons (SoR/GBomb being your best bet) or Draik MPs/Eggs then gift out to shells.
Some reccomend shop sniping, other's don't.
Move them through multiple shells using proxies and you'll more than likely secure it.
2 months minimum for it to be considered safe.
Ive had premium items freeze on me after 2 months so you can't always be 100% still.