Open a new A5 size (210mm x 148.5mm) white canvas.
Add text and graphics (clip-art that suits your text) for your greeting card.
Pay attention to visual hierarchy -- the size of the headline, subhead and body text should be different font sizes and different fonts for the correct emphasis and visual balance. This is the hardest part and lots of practise.
Find royalty-free images from the Internet that you may use.
Finish your A5 size greeting card design, export as PNG.
Create a new A4 size (210mm x 297mm) white canvas.
Load the A5 image into this A4 page if you intend to print it on A4 paper and fold to make a card.
Create the cover of your card in A5 size (210mm x 148.5mm).
Source: https://www.ubuy.co.id/en/product/2QAI2HQS-best-paper-greetings-144-happy-birthday-cards-assortment-with-envelopes-18-colorful-designs-for-men-