Go to Polygons

Add a new polygon
Select the lines or tracks you want to buffer

Select Buffer

Fill out the form

Buffer width is on each side so total buffer will be 50 m
Save
Buffer is created and you can edit it

Go to Polygons
Add a new polygon
Select the lines or tracks you want to buffer
Select Buffer
Fill out the form
Buffer width is on each side so total buffer will be 50 m
Save
Buffer is created and you can edit it