There are some good examples above - Let me share some techniques that I use when planning both VFR/IFR flight in sim, Some of this techniques I use in real life when planning for a flight.
For VFR flying - I normally start by choosing departure and destination aerodromes then plotting a route on chart (Plan G/SkyDemon/CAA 1:500 000 VFR Chart) The last two I use it in real life. Things to consider when planning VFR flights is types of airspaces the route goes through. There are different types airspace classifications from Class A to G. As VFR, Class A airspace is restricted so have to avoid at all cost as the only flights permitted in class A are IFR flights. You will need a clearance before entering class B to D airspace as those are known as controlled airspace. Class E-G is uncontrolled airspace this is where normally majority of VFR pilots operate. Based on this information you then select appropriate cruising altitude, ensuring that you stay clear of any controlled airspace and also clear of any high obstacles (buildings/towers/powerlines etc) in the path. You can plan to fly through controlled airspace (Class B-D) but you will need to request clearance before entering. Looking at Notams (Notices to Airmen) for any restricted airspace/airport facilities/Nav Aids serviceability) To be honest Notams don’t matter in flight sim unless you are flying on Vatsim. Next big thing to consider is weather of course. As VFR, You stay clear of cloud at all times so have to ensure that you don’t fly into cloud. I normally fly VFR in the UK so I use met office low level weather forecast (Aviation Briefing Service - Guidance - Met Office) to have a look at forecasted weather to look for any trends, winds, pressure, any weather fronts affecting the flight and the biggest threat of all ice. If low clouds/visibility are forecasted then I would elect to fly IFR instead. I would also consider choosing alternate destination aerodrome should the weather become unsuitable at my original destination. Using all of this information then I create a navigation log or plog as they like to call it. This includes headings I need to fly, altitude, forecasted winds, estimated ground speed, time en-route, communication frequencies and fuel burn. I use Sky demon in real life for Plog or sometimes I do it the old fashion way by using flight computer. Now as you can see it is long process and in real life it can take me anywhere from 45 to 60 mins to plan a flight. Of course for flight sim you don’t have to complete all of the steps and some things do not apply but if flying on networks like vatsim, I would at least check airspaces around so that I don’t bother any controllers. You can fly without planned destination aerodrome but still have to consider things above.
Of course another thing to consider is weight and balance (distribution of weights and centre of gravity location) and in flight sim is not much of an issue unless again if you want to be realistic. You can use fuel and payload options in sim to change any figures. In real life it is bit more than that.
For IFR flights, Simbrief is best source for planning flights in flight sim. I would be a little cautious with the routes that it gives as sometimes it is not accurate or does not comply with ATC. https://skyvector.com/ - Sky vector is another good source to check routes and I use it real life for planning IFR flights. Again if you want to be realistic then you can check for Notams/Weather/fuel consideration - This is included in flight plan created in simbreif. Another thing to consider for IFR flights is SIDS (Standard instrument departure routes) and STARS (Standard terminal arrival routes)and in real life at least in Europe and UK they are assigned by ATC. I like to use Navigraph for departure, arrival and en route charts and I must say they are accurate to the real charts. I have Jeppesen subscription which I use in real life and found navigraph charts to be quite accurate. Majority of my IFR flights I conduct on vatsim so I go through every process as I would for real flight.
In terms of where and what you want to fly - consider signing up for virtual airlines. operations include from airline flying to bush flying. There are hundreds of them online and sign up process is free. Some of them are run by real world pilots and they even offer training in sim.
Flightradar24 is best website to check for which airlines are operating a specific type and what flights are operated.
Flightaware.com is another one for checking real world routes although this only works for US and Canadian flights.
There is another website https://edi-gla.co.uk/, you will need to create an free account for this and it offers routes flown by real flights in europe and UK.
