There is no lion in the Ledgebay catalog. There is a tiger, and everything that makes a lion difficult to paint applies to it in the same way, so this is worth reading either way.
The big cat we do have

Bamboo Tiger is a tiger's face seen through a screen of bamboo and grass leaves, with the green eyes looking straight out at you. It is one of our best-selling designs, and it is one of the more demanding ones.
The leaves crossing in front of the face are the whole idea of the composition. They are what makes it read as an animal watching you from cover rather than a portrait of a tiger, and they are also the hardest part to paint, because they are long thin regions lying across the values of the face behind them. There is a temptation to simplify them or to let them merge into the background. Do not. Painted properly they are why the design works.
Fur is not painted hair by hair
The first thing people get wrong. A numbered canvas gives you the masses of light and shadow across a coat, not individual hairs, and it should. Filled flatly and cleanly, those masses read as fur from the few feet away where the picture will actually be seen.
Two habits improve it. Pull the brush in the direction the coat lies rather than back and forth, because acrylic keeps a faint bristle texture that will do some of the work for you. And at the outer silhouette, where fur meets background, a very slightly ragged edge reads more like an animal than a perfectly clean one.
That is the one place on the canvas where precision works against you. Everywhere else, keep the edges crisp.
The face carries the likeness
The eye is far more sensitive to error on a face than anywhere else, and that holds for animals as well as people. A tree can be a little wrong and still be a tree. A muzzle a little wrong is a different animal.
So the area around the eyes, nose and mouth deserves the slowest work on the canvas, and it should be painted when you are fresh rather than at the end of a long session. Everything else can be done tired.
The eyes go last
Leave them until everything around them is completely dry, then use the smallest brush you have.
The mark that matters is the catchlight, the small bright reflection in the eye. It is the single most important spot of paint on the whole canvas, because it is what makes the animal look alive rather than stuffed. Keep it small, put it in the same position in both eyes, and put it on last of all.
The same restraint applies to the highlight on the nose. Small, late, and slightly less than feels right.
Whiskers
The thinnest regions on the canvas, and on many designs barely more than a printed line. Paint them after everything around them is fully dry. Use the narrow edge of a flat brush rather than the tip of a round one, because a rigid edge draws a straighter line than a point does, and pull each whisker in a single continuous stroke.
Do not thicken them, and do not correct one halfway along. A whisker painted in two strokes with a join in the middle looks exactly like a whisker painted in two strokes. If one goes wrong, let it dry and paint the background back over it.
Our guide to brush types covers which brush to reach for and when.
If you want an actual lion
Custom Paint by Numbers turns a photograph you send into a numbered canvas, which is the way to get a lion, or any other animal we do not make.
One thing worth saying plainly about that. It has to be a photograph you own. A lion picture found online belongs to whoever took it, and sending someone else's photograph to be printed is not something we can do. A photograph you took at a zoo or on safari is fine, and so is one taken by someone who has given you permission.
For the conversion itself: get as close as the picture allows, choose a frame where the animal is separated from its background, avoid harsh overhead sun that drops the eye sockets into shadow, and make sure the eyes are sharp. If the eyes are soft, nothing else rescues it. The same guidance applies to any animal portrait, and our piece on painting your own dog goes through it in more detail.
The rest of our wildlife designs are songbirds, wolves, swans, bluebirds and a moose, and they sit together in the animal designs.