Start in this order:
Tell the FMC where you are (position init)
Tell the FMC which route you are flying (ie EKEB -EGSS), this is done in the route page.
Tell the FMC which runway you will depart from and which DP (departure procedure) you will use (formerly known as SID, standard intrument departure route). This is done in the DEP ARR page
The FMC now needs to know how the route from EKEB to EGSS will look, this is done in the LEGS page.
Type in these waypoint names
MIKRO- TUSKA -WELGO- JUIST -TEMLU- EEL- PY- GORLO
REFSO -XAMAN - LOGAN -EGSS
Now you may want to use a DP that uses MIKRO as a transition to the high altitude airways. Same thing when choosing the STAR into Stansted, if you have one from LOGAN it is going to be easier for you.
Now we must tell the FMC aircraft perfromance parameters, such as weight, fuel onboard, cost index and cruise altitude etc. This is done in the PERF INIT page.
After that you go to the takepage (we can skip the thrust derate page for now) and type in the takeoff flap setting and the FMC will calculate the V-speeds for you (just click on the buttons corresponding each v-speed).
If you've done everything right, it should say "PRE-FLT COMPLETE
That's it for now, see that was not hard at all 8)
You just have to understand what the FMC needs to know

Oc course you can fiddle with lots more settings to make it as realistic as possible, but the above is enough to get you airborne and to your destination