1/ They should make the race as hard as possible well before the final. Else it doesn't make sense to have Evenepoel attack. He doesn't have the zip or punch to drop other WT riders when they aren't on the limit. Else every Tom, D!ck and Harry could simply get in his wheel and he'll have a hard time getting rid of them. It also makes no sense to attack if the entire peloton is fresh and can get organized easily. If Italy, Netherlands, France, Denmark... all have one or two fresh domestiques in the chase, they can keep him within reach without breaking a sweat. However if the race has taken its toll, and he attacks from a group of 40, with teams being diminished to 2 or 3 guys, getting organized will be a lot harder. In that case, he will force bigger names to commit to the chase, which is in favor of Van Aert.
2/ If he can't get away solo and has some other strong riders with him, he should ride until there is a sizable gap, and then stop cooperating. He doesn't have a sprint, his leader is in the chasing group, he has no incentive to ride once the gap is established. He has nothing to gain from further distancing his leader, and towing rivals to the finish for him to be beaten. If his companions keep riding, he can attack them and try to get away solo after all.
3/ He has to attack before leaving the Flandrien circuit. He can tire his companions out if he takes the time, but not on 10 or 15 km. Having him attack in one of the last local laps makes no sense unless he can go solo, but at that point nobody is going to let him leave and the local laps aren't hard enough for him to get away solo anyway. Attacking from further out will invite b-tier or c-tier favorites to try and go with him, while the biggest guns will probably not feel comfortable going in the offensive too soon, risking to put everything in an attack that has a high likelihood of getting caught, especially with Evenepoel not towing them to the line, like he did Colbrelli two weeks ago.
4/ He doesn't have to attack too soon either. The sooner he attacks, the bigger the peloton will still be, the easier to control him. Having him attack with 100k to go is not credible, even for him, unless he's part of a bigger break initially, that might get some freedom from the peloton, when rival countries assume they have a guy up there who can keep up and outsprint Evenepoel.
So, make the race hard before his attack. It has the added bonus of choking (or dropping) guys like Ewan and getting rid of a lot of domestiques, meaning the bigger names will have to get their hands dirty sooner than they'd have hoped. Attack from further than what most of Van Aert's rivals would deem comfortable, so that they would not try to go with Evenepoel at that point, meaning he doesn't get the cream of the crop in his wheel, but guys he might be able to drop eventually on this course.