How to fix holes and cracks in wood?

In my experience the Elmers wood filler type putty in the tub is the most user friendly.

Recently I talked tried the same product in a smaller amount that eas offered in a tube, and I found it very gritty or sandy. I prefer the tub product consistency and application ease.

http://www.elmers.com/wood-filler

Wood filler does that job. Different types are available. Mosf are stainable. All can be sanded to a smooth finish after they dry. One attached works interior and exterior. Good luck! Carpenter's 32-oz Natural Wood Filler https://www.lowes.com/pd/Carpenter's-32-oz-Natural-Wood-Filler/4086120

I like putting some Elmers white glue on a piece of paper and saw dust mixing it to a paste and filling the hole. Let dry for about 12 to 24 ours sand and paint or stain worked for all my projects and accepts paint or stain well.

I was about to offer the same answer as Arnold Jones. I actually mixed sawdust from the wood floors being installed with some Elmer's white glue and filled in around places next to door facings were there were small gaps. Once it dried, I used some acrylic paint to add touches of color to match the grain in the wood.

Elmers wood filler is great but sometimes it just doesn’t stick. Put one- only one drop of water in, mix, and spread. Sometimes you’ll need to do a second coat and I level mine with a putty knife or old gift card

The best way is to sand the floors save the dust and buy a 1/2 gal of Elmer’s wood glue and fill. That way the color stays the same and it looks wonderful I did a 4000 sq foot house that way turned out great.

I use wood putty or glue for filling holes or cracks in wood. I make wooden boxes and frequently run into this situation. In making the boxes, there is a lot of sawdust that I use to mix with glue and then clamps pulling the crack together. Once dry, you'd never know it had been there. For small holes, wood putty works great AND you can stain it.

You can also watch a few YouTube videos to see if your particular issue is addressed: https://www.youtube.com/results?search_query=how+to+fix+holes+and+cracks+in+wood