I think they need to go into it with the acceptance that errors will happen. And with that knowledge then raising the VAR bar like crazy and only bringing it in for blatantly bad decisions. If a VAR ref can't tell on 1 normal replay that the onfield decision is bad/wrong, VAR shouldn't intervene. If it takes more than 1 or 2 views to make a decision, then on fid decision should stand. If an offside looks tight on a replay, on field decision should stand. If the VAR shows a clear on/offside after a replay, then stop play and have a look (lines should basically never come into play). If a penalty is missed but looks a pen after replay, it should get overturned.
Get VAR in for the really bad calls. Leave everything else down to the refs. Will mean they have to ref normally and can't ignore reffing situations as "var will sort it out" only for the VAR ref to deem it "not a clear and obvious error" and both being on different pages. Refs should ref the game and VAR should only ever come in for blatantly bad or wrong decisions. Stuff like Drogbas offside in 2010, Henry's or Maradona's handball, De Jong not getting sent off for a Kung fu kick in the world cup final, Rashford getting tripped in the Manchester Derby last year but VAR needing to step in for the pen, etc. Blatant calls. Never ever any marginal or tight decisions.