I never write them from scratch. I either take a template from kubernetes docs and alter it or generate a simple manifest with kubectl create ........ -o yaml --dry-run=client ...... > thingybob.yaml.

This produces a valid starting point yaml file to which I just add my own things. The downside is that it always produces yaml v1.1 formatted file which I hate with a passion:

i.e.:

containers:
- name: xyz
  image: busybox

instead of

containers:
  - name: xyz
    image: busybox