Routine help at 18 weeks old!

I'm having some problems getting my DD into some sort of routine.

Just wondering how you all established a routine with your bubs.

First thing I would suggest is not to watch the clock - having a routine for a baby is not about doing things at the same time every day because if that's what your aiming for you'll just drive yourself nuts.
With both of my babies I have always made a point of just doing things in the same sort of order every day so they know what's coming next. Some days it might be two hours earlier, other days might be three hours later but the order of things generally runs the same. So for my DS (who is four months old) this is how his day generally runs:

6.30ish - wakes up, breastfeed
Awake time
Morning nap (usually about 2hrs)
Awake time
Bath (usually happens early to mid afternoon)
AFternoon nap
Awake time, catnaps downstairs in the early evening
7.30ish - last feed and bedtime

He demand feeds during the day so I haven't included his feeds above.
Think about how your day normally goes, and the things you need to do and try to sketch something out.
It truly is trial and error and some days will go better than others but I have found both my kids have been much easier to handle and sleep much better when there is some predictability in their day.
Good luck :)

We just adapted to a routine over time.. it generally just went on his feeds and when he woke in the morning.. now he routinely wakes at 6am so the day goes from there. But no matter what time he's fed during the day (usually every 4 hours), i always feed him at 5.30pm and then bath him.. then feed again at 7.30pm and he's in bed by 8pm. I just inforced that gradually and he adapted to it easily enough.

We just follow the feed, play, sleep routine. No set times just go by if she is tired and ready for a sleep. Generally I find she will sleep roughly at similar times but will vary on the time she gets up to start the day.
What you could do is chart what she does for a week such as the times she feeds and sleeps and see if there is some sort of pattern.