Sand well first, pick a color in a gel stain, wipe or brush on, wipe off . Let dry overnight, apply a few coats of varnish, lightly sand between coats with 220 grit sand paper.
If exterior must use spar varnish on top.
I second Kellis directions
Wipe it down really well with a tack cloth, then use prestain conditioner to make sure the stain goes on evenly without splotches. Then stain with wood stain. You can add more coats after dry time to make it darker.
Once the stain has dried for 24 hours, seal in the wood with oil based polyurethane.
This is a great article https://homeguides.sfgate.com/stain-interior-door-22704.html