How can I gain any authority?

[SirRaymondClench]

I am married to DH and I have two DC (14 and 8) from a previous marriage. DH has DC who lives with us (13) so we are a fairly big household.
DSS will do what is required of him ie get in the shower, do homework etc but my two seem to push it to the limit so we end up telling them to do stuff 10 times over. It's so draining.
They are better when DH asks them but still can be dragging their feet but the problem is mainly me.
I seem to really struggle to gain any authority because I am deemed 'too soft'.
I think I am starting to realise how shit I am at parenting.
They aren't like this with their dad who Disney parents them on the odd weekend he can be arsed having them.
Please give me some tips on how not to be walked all over.
Shouting seems to do little apart from give me a sore throat.

Stop telling them to do stuff 10 times. Ask them once, then give them a warning that if it's not done in 5 minutes there'll be a consequence/punishment. Make sure you follow through with the punishment. They'll soon learn you mean business.

Consequences can be better than punishments IYSWIM as they learn the effects of their actions or lack of action. So if they are slow to get ready to go out then you won't have time to go to the shops they want to.

Always follow through, don't enter into debates with them and stay firm.

I'm going to try that Fernie
I had major surgery last year and have had ongoing problems so haven't been my best for a while and they seem to have taken advantage of that weakness and stepped it up a gear!

Teens love winding adults up and stringing out arguments.

State your request, state the consequences for not complying, walk away. Invoke the consequences firmly if X didnt happen on time. Removal of phone/mp3/tablet/laptop for a day/evening/weekend is usually the most effective 'punishment'. Stay calm, talk quietly. You are in control, not them.

It'll take a few times for it to sink in that you really do mean it. But it DOES work.