The problem with growing fruit trees from seeds is that it may take very long to bear fruits - some times more than ten years - and there is no guarantee that the fruits will taste same as the one from which you got the seeds.
Air-layering is one method of propagating a fruit tree from an existing one, which will bear fruits sooner, and the fruits will taste same as the mother-plant.
Most of the Citrus trees can be propagated by air-layering. I have successfully air-layered the sweet lemon tree at our home garden and here are the step-by-step instructions on how to do air-layering.
Step 1: Tools and material required
1. A sharp knife
2. Thick cotton thread
3. Clean plastic sheet
4. Hack saw blade (or) a Pruning shears