The first question is do I think a side will keep a clean sheet? If the answer's yes then I'm always selecting that team's best defender and their goalkeeper. Goalkeepers score huge if they don't concede and are a must in this scenario. Be warned though, if the team does concede and you have their keeper plus at least one defender, you're in trouble!
If I'm not expecting a clean sheet on either side then the next question is do I see an obvious winner? Arsenal at home to a poor side is a good example here, they'll often concede but also will score 3 or 4 themselves and win the game comfortably. In this case I'll select all the Arsenal attacking threats I can, trying to ensure I get every single one of Arsenal's goals and assists. If the tourney decrees I must take a defender too I'll generally take the most attacking one e.g. Kolasinac or Bellerin.
If there's no clear winner and I don't expect a clean sheet on either side then I'll generally try and take the key attacking players from both sides. For example if Bournemouth host Leicester there's a great chance of goals from both teams, they're fluent going forward but shaky defensively. Therefore in that game I'm likely to have a lineup that's a combo of the two team's key midfield and attacking assets. If I have to take a defender I'll take the most attacking one from whichever team they play for, in this example probably Rico or Pereira.