Depends on how realistic I'm trying to be.
The computer GMs in this game are all really, really stupid.
In Legacy, it's possible to completely cheese your way through with 3 simple steps:
1) Scorched earth rebuild in Y1. Anybody older than 23 with enough trade value is out the door. Pick up as many 1sts as you can, and try to identify which teams are lottery teams. (the Y1 draft is always really deep potential-wise, so going for 2nds and even 3rds is an option too) Keep your own picks (make sure you have your own 1st) and tank hard. Keep your younger, high-potential players, though, and make sure they're playing in the correct roles (so if you have a 3rd liner with Elite potential, play him on the 3rd line and have all your AHL scrubs and goons be the fillers in your top 6)
2) Prospect cycling. Identify which prospects you want to keep for the long-haul, and which ones aren't going to fit in with your team. Move those prospects for the next year's 1st round picks from the worst teams. A tip for Legacy -- there's something with 4.5 gold star players called the "trade value curse". Basically, you can tell whether or not a 4.5 gold star player is likely going to become a ~90 or 90+ player by simply looking at their trade value. Players that will become elite will have trade value at about 2/3 to 3/4 of the trade bar, while players that likely won't (typically drafted from 3rd-5th overall) will have much lower trade value. (about 1/3 of the trade bar) Those prospects might still become solid 2nd line forwards (they typically cap at around 85-86), but in the terms of abandoning realism, their trade value is worth about the same as a potential top 1st round pick at the beginning of the season. It's always better to continually cycle these decent prospects because they can give you a chance at getting a 5 gold or even green star player at the top of the next draft.
3) Shed older players before they start dropping. If you're doing this all right, you should have a steady stream of new players becoming ready for NHL duties. Once these players hit about 30 years of age, they start dropping and so does their trade value. Make sure the hole they'll leave in the lineup is filled, and move them out for 1st round picks.
Obviously this is unrealistic as hell, but it works really well on Legacy and NHL 15. (and sort of on NHL 14) I had a team hit 76 wins one season about 10 years in using this "strategy". (although cap was off to be fair) All sim.
The only free agents I pick up are prospects that teams let drop to FA.
Not sure how well this works on NHL 17 (since I know EA added team status bars and you'd think that a team proclaiming itself as a "rebuilder" would be a little smarter about moving their 1st round picks) but this works like a dream on old-gen.