To tar rope, use natural fibre rope, apply a controlled amount of suitable tar, and work it into the fibres so the rope becomes darker, more weather-resistant, and less prone to surface rot.
Tarred rope is a traditional material for maritime, outdoor, historical, and decorative uses. It is most relevant to hemp, flax, manila, jute, and similar natural fibres. Modern synthetic rope is usually not a good candidate because tar may not bond well, can interfere with inspection, and may weaken or contaminate rope used for safety-critical purposes.
The simplest craft method is to use a small amount of pine tar or rope tar and work it along the rope in sections. The aim is to coat the fibres, not to bury the rope in a thick black mass. Twisting and flexing the rope gently can help the tar reach surface strands, while wiping off excess keeps it from becoming unmanageable.
Tarred rope becomes stiffer, heavier, darker, smellier, and more likely to stain hands or pale surfaces. That is part of its traditional character, but it limits where it should be used. It suits display rigging, garden work, rustic handles, reenactment kit, and craft projects better than climbing, lifting, rescue, or clean indoor use.