don't worry, its a short drive from miri to lawas, just make sure you have the following:
1. car fuel is enough - fill in malaysia side; if you fill in brunei side; you will pay the marked-up price at a special pump; dont do it unless emergency
2. car registration for brunei custom done online nowadays - save the QR code onto your phone; via screenshot and put your phone screen brightness to max when scanning at border later
3. toll is BND/SGD 3 at kuala belait
4. if you are not sarawakian; make sure to bring the sarawak entry slip or the dokumen perjalanan terhad along to exit malaysia border
5. car tint is not allowed in brunei - depends on your luck - summon is 50 BND; and yes... even the default new car UV tint also will kena... as long as you wind down the window and got a tint layer, police will summon... all brunei car have ZERO tint... except for the "special" folks

pm me if you need more info... enjoy the drive!

hahahaha u have better luck than me i guess... my 4wd only got the default UV tint... 50 bnd gone
the check-point is in between KB and Seria though... on the main highway