I used to suggest the method I used to use, where you find or create a database entry, add that to Your PLANT List, and then form there, add it to Your SWAP List, and from there, add it to THIS Swap's Tradelist (The Green Check Mark).
But that's the HARD way!
If you can use this swap or any swap's " Manage your own swap list " button, that lets you type a name directly into a box, directly into Your SWAP List, and save many steps.
The above method is what Dave teaches in his YouTube video.
If you let us know where you hit roadblocks, we can give more suggestions.
The video is Step One, and joining a swap is Step Two.
If it is too late to join us, there are two other NGA swaps currently gathering members:
http://garden.org/apps/swap/
They all use the same swap tool, so anything you learn trying to join this swap will help you with the next swap.
Here's a sweet feature: once you have created "Your SWAP List", you can re-use that same list for as many future swaps as you want.
You just have to go to the page that has the green or gray check-marks, and check the checkmark to add THAT entry to THAT swap's Tradelist. (And you probably have to adjust the number offered to the second swap, if you gave some of them away during the first swap or used them up yourself.)
)
with Rick @profesora you can still join The 2nd Annual "Not from the Seed Rack Seed Swap" 
