How inportanant is to clean from lead fouling built up on the forcing cone and how properly can be donne..

The main consideration would be the cylinder to barrel gap can easily retain a powder residue build up resulting in the cylinder locking up or major resistance in turning. Depending on the primary use of the gun, this would be bad juju in a self defense gun. The solvents generally available and brushes can easily stay on top of that, some ammo would certainly foul worse than others.

The easiest way to clean lead deposits off that area is to shoot a couple of cylinders of hot jacketed rounds. Powder residue isn't much of a problem. Lead buildup is. One thing to check is endshake. If the cylinder can move front to back, it makes any leading problem worse. While you're at it, check the timing and barrel to cylinder alignment. If these are off, it can contribute to lead shaving. Additionally, you can spray Dillon case lube on the forcing cone and front of cylinder every 50 shots or so. This will make any deposits not build up as much and easier to clean off what does build up.

If the fc is considerably leaded, you might also encounter unusual high pressure signs.

I recently was having a fit loading strong .38 Spl loads in my .38/44 Outdoorsman. Marginal loads were showing some evidence of high pressure.

After failed attempts to solve the problem, I properly cleaned the fc with brush and chore boy. Bingo--pressures dropped.

Seems leading is harder to see on the fc than the bore.

Revolver forcing cones can not only get lead build up they can also get fouled with copper and carbon.
One possible contributor to the infamous cracked S&W "K" frame forcing cones was that many were badly fouled.

There's one sure and certain way to clean carbon, copper, or lead off a forcing cone and do is FAST is to buy a Lewis Lead Remover kit from Brownell's.
All the older revolver shooters had a Lewis Kit.

In the kit is a special cone-shaped tip that uses a brass screen to cut any fouling off the cone.
This takes only one or two turns and it's clean.
Even if you shoot only jacketed bullets you still should use the Lewis Kit.

BROWNELLS LEWIS LEAD REMOVER | Brownells

Personally I don't think it's really all that important. The only function the cone serves it to guide/force the bullet into the bore of the barrel. If the cylinder isn't out of alignment with the bore you'll get very little to no leading of the cone. If you're getting lots of leading in your forcing cone you should probably have the cylinder to bore alignment at lock up checked out. Severe leading implies that at lockup one or more, possibly all your cylinder throats aren't in perfect alignment with the bore. About the only time I bother with cleaning lead buildup out of my forcing cone is if it will be put into long term storage.

The Lewis Lead Remover (from Brownell's) is excellent for cleaning up lead and copper fowling from the Forcing Cone as well as the Cylinder Chambers and Barrels of Revolvers. A stiff Nylon brush is also great for cleaning around the outside of the Forcing Cone under and over it inside the Frame and under the Top Strap. I like to keep my guns clean and this is the best method I know of for the FC and immediate area.