Silver cross simplicity car seat help!!!

So I have the silver cross wayfarer pushchair and simplicity car seat.. I don't have an isofix base but know I can use the cars seat belt.. does it sit flat to the car seat? The seat hasn't got the level lines on to no when it's in the right position before I secure it with the seat belt? Does anyone else have this and can offer any sort of advise to make sure I use it properly??? Any help will be appreciated