Williams have won 9 Constructors' championships since they entered the sport in 1977. That is second to Ferrari in the history of F1, a remarkable feat for a privateer team that doesn't have the backing of a major manufacturer or even a global drinks company. The last of those championships came in 1997 under the guidance of Jacques Villeneuve and Heinz-Harald Frentzen. Villeneuve also won the Drivers' title that year, Williams' seventh, and their fourth of the 90's.
Since then things have gone poorly for Williams though. Ferrari soared back up the field. Red Bull entered F1 with their remarkable financial strength, and Mercedes came back in as a full-fledged team not just an engine provider. Williams have won 114 Grand Prix in their history, with the last one coming in Spain in 2012. That result was extremely unlikely, and since then their chances of standing on the top step on a Sunday afternoon have only reduced. In 2017 they finished 5th in the Constructors' championship, and in 2018 they were dead last with just 7 points to their name. In F1 2018 this downward spiral needs halting, can you be the one to do it?
Williams are the worst car on the grid on 2018, and by a substantial margin. Because of that their expectations are simply to "Be Consistent". Basically, don't put it in the wall and try not to finish last. That's not asking a lot. The biggest issue you'll face with Williams is a complete lack of competitiveness early on. You'll be left behind by every team very quickly, leaving you with just your teammate to compete against for a lot of the early races. If you can handle that frustration then you can lead Williams back up the grid eventually.
The Williams Racing car is the worst one on the grid, and it isn't particularly close. They sit dead last in the chassis department and are tied for last in aerodynamics. The one saving grace the Williams has is the Mercedes power unit which puts them third, but with their unbalanced car and inefficient aero package it can be very twitchy coming out of the corners early on.
You get a nice 1,500 resource points when you start your career with Williams, and these should be put straight into an efficiency booster and chassis upgrade to help the balance and make the car more predictable for you under braking and during changes of direction.
Bringing Williams up to the midfield battle is not an easy task, but if you want to get them back to the front then it is a long, hard grind of practice programs. Improving the chassis and aerodynamics is the top priority for Williams, but you shouldn't ignore the powertrain either. With circuits like Azerbaijan, Spa, Monza, and Russia on the calendar you need to improve your power eventually, but the chassis and aero upgrades will make the car far more driveable in the short term.
The quickest route to a major upgrade is in the chassis department, which is an interconnecting maze, but the aero department offers the most linear upgrades. The closest ultimate upgrade is in the powertrain department, but that is another good reason to focus on the other two areas first and get them up to scratch so you can win some points and rake in resource points even faster.
If you aren't prepared for a multi-season journey, then Williams are not the team for you. This is not a team you can win a championship with immediately, or even necessarily in season 2 if you set the difficulty high enough. However, that means by the time you do claim the title it will be all the sweeter. Since expectations are so low the early poor results won't lower your reputation with the team, and once the results improve, they will love you. You may not be able to bridge the gap to the big three teams for a long time, but you'd be surprised by how quickly they can fall back to the midfield and be overtaken by a Renault or Haas.
Your best chance for an early result is likely to be Monaco, where you can strategize your way to the front of the pack with a well-timed one-stop strategy. Otherwise it is a long road to being competitive. At top end speeds the Williams is pretty good, but carrying that speed through corners with any predictability is impossible. The real life Williams team are in a horrible hole right now, but you can play the hero in F1 2018 and restore this team to greatness if you are willing to put in the time.