Seasoning wood depends on the species and the size of the pieces. Some species disintegrate when submerged (grape wood). Some are unsafe for aquariums (cedar, lilac, yew).
Generally, you should leave it dry, open to the elements for a while to properly "cure". Remove the bark as much as you can during the process. This can take weeks or months, it depends on the size, species and the conditions where you live. When it's fully dry, it will be much lighter and sound hollow. At that point, you can submerge it.
Change the water daily at first, for as long as it's changing the water appearance in that time frame. For me, the daily water changes lasted about a month with small silver maple branches. Then it was every other day for about 2 weeks. Then once a week for another month. Then I scrubbed at it and let it dry out in the elements again for another week or so.
Then I boiled it just to be safe, but that's not really mandatory.