Shown here are the most popular routes to reach Halti. "Retkeilykeskus" (Camping Center) is the beginning point in the town of Kilpisjärvi (just above the big lake). This is where all the services and stores are. The normal route, called "the Halti Highway" is shown in red and runs via Saarijärvi, Meekonjärvi, Pitsusjärvi (this is the lake you can fly to) and finally, Halti. You can come back via the Norwegian Route, but note that all the huts are on the Finnish side. DO NOT mistake "Urtas Hotelli" for a hotel, it's actually just a big box, where 2-3 people can sleep.
The triangles depic peaks (with elevation in meters) and the squares huts. It typically takes 3 days to trek the 55km route, roundtrip in maybe five days, depending on how fast you go. I would recommend 15-20km per day - anything over that is really pushing it, and you donn't enjoy it as much.