What you can't work it out???
Same in any FMC.
Copy the active.
Modify the sec to another destination from a Lat revision much further down the track after where you estimate the PNR should be.
Link up Airways/Waypoints back to the PNR Airport including the RWY and STAR arrival.
Check planned FL's, entered winds etc.
Look at the Arrival fuel at the PNR, it's up to you how much fuel you would like to arrive at the PNR with. I'd suggest a minimum of 30 mins but consider this does not yet take into account a Depress cruise.
If too much fuel then move the PNR further out
If not enough then bring it back a bit.
This isn't too hard and works ok.
Remember to "fine tune" all the inputs into the box as much as you can for a more accurate answer of FOB at the PNR.