A good road to travel from Stockholm is the E4 via Gävle towards Bollnäs-Ljusdal-Sveg-Funäsdalen-Tännäskröket.
When travelling from Gothenburg drive via Uddevalla/Trollhättan - Sunne - Malung - Särna -Tännäs.
The direct bus Härjedalingen departs regularly all the year round from Stockholm, Uppsala and GävIe. From Gothenburg simply take the night train via Östersund and then bus. Resplus (train and bus) works well throughout the country.
Direct flight from Stockholm to Tännäskröket via Röros. Travel from Arlanda to Tännäs in approx 2 hours including transfer.
You can fly in to Sveg (NextJet). SAS flies to and from Östersund. You can also hire a car in Sveg and Östersund.