Dense-packed cellulose is the best insulation to use for walls. It is inexpensive and packs tight enough to prevent air movement through the wall. It not only makes the wall warmer in the winter but also makes the house less drafty.
Holes are drilled every 16-inches to access the wall cavities. Most often this is done from the outside. The siding is removed, holes drilled, cellulose dense-packed into the walls, the holes are plugged, and the siding put back in place.
If you are planning on painting inside, the holes can be drilled through the plaster or drywall and patched after the insulation is installed. This may be the only way to access the wall cavities if your home has stucco or other masonry on the exterior.
Dense-packing cellulose requires special equipment that can generate the pressure needed to pack the wall cavity. The equipment you can rent at home centers should never be used for wall insulation. The insulation will be loose allowing air to move and moisture to condense inside the walls. Hire a professional!