First you'll need to determine if you are collecting "wet" or "dry" seeds. What are "wet" or "dry" seeds? Dry seeds are typically found in a pod or stalk, wet are found in the pulp of a fruit or vegetable.
Here are a few examples!
| Wet | Dry |
|---|---|
| tomato | peas |
| eggplant | beans |
| squash | sunflowers |
| pumpkin | corn |
Wet seeds should be cleaned of any pulp and placed in a jar with a small amount of water for several days. Stir several times daily, viable seeds will sink to the bottom. After a few days, pour out the water, and set them out to dry.
Dry seeds should be collected when the pods are dried, remove from the pods and spread them out in a dark, dry location.
Seeds should be stored in a dry and cool location in glass or paper.
Source: https://extension.unh.edu/blog/2020/10/how-do-i-save-seeds-next-years-garden