Is she penned up? You will most probably have to assist the birth, so rather than chasing her round when she's in labour, it would be easier to have her already inside. Do you have someone who can help who's experienced? Keep a look out for snot from her back end, a single small strand can indicated labour in 2 hours to 2 days!, more than that then its happening! But the absence of snot doesn't mean she's not in labour!
Have ready plenty of lambing lubricant, or we prefer Lux type soap flakes in warm water. There should be lots of threads on here about correct presentation and what to do ( Sally in the North usually gives very good explanations which has saved many a lamb of ours!! - thanks Sally). With a big texel single lamb you can have a correct presentation (2 feet and a nose) but it still get stuck on the shoulders. If that's the case, if the head is firmly in the canal and no chance of slipping back, pull one leg out so its fully forwards (think superman) and then pull the other one fully forwards. Making sure first that there's lots of lubricant around the head as far back as you can, pull both legs with a firm pressure. You may get to a stage where the nose is out and the head appears too big for the opening - outside of her, try to get your hand around behind the head and squeeze at the same time as pulling the legs to pop the head through the opening. I've heard of people putting 2 fingers up the anus to push from there but you need to be careful not to damage her (no long finger nails!)
edited to add - sorry you did say she was inside!