My tip is in your question... DIY! LOL
Seriously though, Willynelsons tip is great.
I made some fitted wardrobes and I made the frame first and cut the panelling out to fit the frame.
The doors were something that I almost messed up right away and I had to make sure that the columns were positioned for thos, but also one door had to be made a little narrower too, by about 3 inches... I made them all anyway, and the narrow door is in the corner so its unnoticeable.
I made the frame square, and used skirting boards at the top to hide the crappy work, and it made it look a nice deliberate feature, so that was a bonus.
I also made a silly mistake in that the way the doors open.. The end ones opens up awkwardly and you have to shut one before you open the other, but again, this is in the corner so forgive me on that.
I can provide pics if you fancy a laugh, but then again, I did them 12 years ago and they are still there so I did something right.