DIY, craft, materials, and such / Dyeing
Cellulose fiber fabrics
Linen is a cellulose fiber, as is cotton, hemp and rayon.
These can be stained moderately well with generic dyes like Rit or Dylon or such, but fiber-reactive dyes are preferred for more permanent and vivid colors. Example: Jacquard's Procion MX.
Check what fabric you have - if the majority is fiber fabrics like linen and cotton, it'll be fine. If it's linen plus something artificial it may not dye so well.
It helps to wash before dyeing - preferably in water, as some detergents have ingredients that may interfere with the dye. (TODO: figure out which - basic detergents seem to be okay (and it can even help to add them for an even dye). Fabric softeners seem to be a culprit)
Prepare the dye bath according to the instructions, dump fabric in. For powder dyes, you can consider mixing it with a little hot water to ensure a good dissolve (to avoid dye spotting), then mix into more colder water.
Avoid bunching. If your fabric went in dry, then you may need to do a bunch more work to make sure it becomes wet everywhere.
To get even coloring, you need lots of movement. This is why the in-the-washing-machine stuff can work works pretty better than occasionally poking things with a spoon. (top loaders can be a little easier to work with, mostly because of their water level)
It can be hard to get darker colors.
Silk and Wool often need an acidic dye bath for good results.