Cupboard Pelmets

What are you using for a mitre guide ? Is it that the pelmets are awkwardly shaped and don't sit squarely ?

If you are really stuck, take the pelmet to your local woodworkers and they'll probably mitre them accurately for you for a few quid.

You can use a mitre block. I used a chop saw, perfect but not the cheap option.

But the problem might be in the way you're fixing the 2 pieces together. Forget using an angle bracket.

If it's MDF use a product called Mitre FFFast. I just installed my pelmets and frustratingly had big problems getting them aligned and looking neat. This glue is a 2 piece kit, on 1 piece you spray an applicator and the 2 nd piece you use some glue drops. Hold them together and after 10 seconds it's stuck. It saved my sanity.

I must say that if you're using cheap laminate MDF types, like I did, there will be some sort of seam. Mainly because the laminate was so thin.

HTH