Left click anywhere on the line that contains the desired letter to copy it. Middle click on the line where you want to paste it. As additional letters are pasted they will appear to the left of previous ones. Note: if the picker does not appear your browser is blocking it. Look in the location bar for a symbol indicating an error and click it. It should offer an option to allow popups from this site.
The font used in the letter picker is Paskol from
the Open Siddur Project ✍︎ פְּרוֹיֶּקט הַסִּדּוּר הַפָּתוּחַ
You may close this page after the app loads
by clicking on the page then pressing ctrl-w