Here is a list of general tips when making a Google Post:
1: Keep your posts within 80 to 100 words.
Even though you can write up to 300 words, not all of the content will display without the end user taking extra actions
2: Following best SEO practices.
Keywords and the most important info should be at the top of the post
3: Use the recommended size for your images.
The best image size is 750 by 750 and the image should be centered so it looks good if it gets cropped. Do not upload an image smaller than 250 by 250 or it will be rejected by google. A happy compromise on size is using a 400 by 300 image.
4: Images with text might be cut off.
Text on mobile devices might be cut off. It's best to stick to images with no text on them.
5: A post is your opportunity to talk directly with consumers.
It's best to use a real world conversational tone in your posts.
6: make sure your landing page makes sense.
If your post is talking about a promotion on cars, don't send your clients to a page selling trucks.
Read more about Google Post recommendations here.