If your woods are short on snags (standing dead trees), you can create some of this valuable wildlife habitat by girdling trees. Girdling kills a tree while leaving it standing so it can become a snag.
Snags are standing dead trees. This article from the Cornell Lab of Ornithology explains their importance for wildlife and gives tips on what kinds of trees to consider for girdling to create good snag habitat.
Snags provide crucial wildlife habitat, but no one wants a forest full of dead trees. This brief MyWoodlot article recommends snag numbers from wildlife biologists based on tree size, because different animals use different sized snags.
Perhaps the best known visual signs of old-growth forests are their truly enormous trees. You can accelerate large tree growth in your woods by finding your healthiest “legacy trees” and removing competing trees around them so the big trees can grow faster.
True old-growth forests have greenery at every level from the forest floor to the top of the canopy. You can start some new growth on the ground using small patch cuts, which remove every tree from an area less than an acre.