Keep in mind you can use stems from angelica, lovage, fennel or any other edible member of the Apiaceae family. Be careful abut your identification if you are foraging, however, as poison hemlock is in this family!
Cut your angelica stems to a length that will fit into the jar you plan on storing them in. Bring a pot of water to a boil and add the baking soda. Get a bowl of ice water ready. Boil the stems for 5 minutes, then shock in the ice water.
Meanwhile, bring the water and syrup to a boil. Get a glass or other non-reactive container ready for the stems and syrup. When the angelica is ready, put the stems in the container and pour the hot syrup over them. Let this come to room temperature, then cover. Let this sit at room temperature overnight.
Next day pour the syrup into a pot. Bring it to a boil and add the angelica stems. Boil for 1 to 2 minutes, then pour everything back into the container. Let this come to room temperature, cover, then store overnight again. Do this once or twice more.
After the final syrup boil, set the stalks on a rack to cool and dry a bit. When they are room temperature, roll them in granulated or caster sugar and store in a jar.
Notes
Don't toss the syrup! It's amazing with a bit of seltzer water, plus gin or vodka if you want an easy cocktail.