Firstly, it’s best to be 4 players doing this, as having fewer players makes it more difficult and time-consuming although it is still possible with 1 or 2. Be prepared to spend hours doing this as it’s not an especially quick (or scientific) process. Also, make sure you have field radios as players maybe “locked out” and these will be essential for access in these circumstances.
You need to head to Östervik (preferably around the time the sun sets in the game) and cause as much destruction as possible. Blow up cars, shoot all junction boxes on every house, blow the gas canisters in warehouses, kill all enemies and ticks etc.
As you do this, search the bottom floor of every house. Then once you’ve gone around all of the district’s (Haga is the most frequent hot-spot) repeat the search over and over and over.
To avoid frustration, and when Östervik is depopulated, players can head out to action hotspots (Hagaboda, Farela, Bockhyttan etc.) and wreak havoc but I’d keep at least one person in Östervik looping and then periodically bring the other players back to help loop / search.
If someone finds that a previously searched house is now suddenly closed, locked down and inaccessible with its junction box somehow repaired, you’re in luck.
Note: it may be that you can get inside and see the gnomes, but a friend cannot.
Those players who can still get inside the house should place a field radio to allow those locked out access.
Also, it’s possible that while searching the area one of you will see a “pick up” icon (the black circle with a white hand) at a great distance away. If this ever shows up, make a bee-line for it immediately as this is usually a gnome.
(More often you’ll get the empty circle associated with light switches, but alas this doesn’t usually point to a gnome!)
Once you find a gnome take up position and shoot the gnomes sat on the floor inside and they will hopefully spawn gnome ticks. Kill those ticks for sweet loot and a chance to receive a mask.
I’d recommend one person does the shooting and uses a single shot weapon such as the Exp Klaucke or a 12G Shotgun. Take your time and try to avoid having the gnome-tick bodies stack up as you can get some strange physics going on and crash the game.