How do I make my business stationery look professional without a designer?

You do not need a designer to get professional stationery, you need consistency. A few disciplined choices, applied the same way across every piece, beat a complicated design almost every time.

• Start from a template: it handles spacing, bleed, and margins for you.
• Stick to your brand colors: two or three at most, used the same way each time.
• Use one clean font: resist mixing several typefaces.
• Repeat the layout logic: put the logo and contact block in the same place on every piece.

Consistency signals professionalism, so a simple design executed cleanly will always look more credible than a busy one.