What is the best method of attaching shells to wood letters?

https://www.thriftyfun.com/Using-the-Right-Glue-for-Seashell-Crafts.html

If your work doesn't have to be outside, E-6000 glue is great, but it does smell for a couple days. Gorilla glue is good too, but begins to yellow over time. This will give you the best information and you can choose the one you think will work.

https://www.google.com/search?q=best+glue+for+all+surfaces&rlz=1C1BLWB_enUS700US700&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjj8cm-o4zYAhUC6GMKHVAVD04Q_AUICygC&biw=1422&bih=684&dpr=1.13

E600. My go to glue for projects. It is clear which will make it perfect. Use in a ventilated space because it using a lot of it the smell is strong.

there is a glue I love and it works with just about anything. It is great for outdoor. Glass glue

It should be. Is there a way you can test one letter and go from there? Here is another good site. When you are done, can you post?

https://www.google.com/search?q=best+glue+for+attaching+shells+to+wood+outside&rlz=1C1BLWB_enUS700US700&source=univ&tbm=shop&tbo=u&sa=X&ved=0ahUKEwi336Pqy4zYAhUJ2mMKHSfmARcQsxgIKg&biw=1422&bih=629&dpr=1.13