Gathering Willow Wood requires a minimum Woodcutting skill level of 8. There is no skill minimum for Marsh Fern.
The nearest place you can harvest Willow Wood is the fields to the west of Gurla's Caravan.
Yield is roughly 1:2 so you are going to need at least 10 Marsh Fern and 10 Willow (Rdb) to make 5 Yellow Juice (Rdb). Better to collect more to cover failures than to have to go get more!