Peening the tang over the pommel does work but you need four hands, two to apply pressure to the parts to hold them in snug position.

I use an 45° 1/4 inch wide chisel to displace metal of the guard against the blade. Has a neater appearance. Some brass guards have had a X shape punch do the same function. It prevents the brass from shearing off if punch is too close to the edge.

The welding shown is actually a poor brazing job, parts were not heated sufficiently. Leave brazing for the muffler repair shop.
Silver Solder works much better and flows in between the parts rather than blob around the outside. Once the parts are at proper temperature with flux applied it takes about 2 seconds to run the silver solder into the joint. Heat stop can be used so no high heat damages the temper. No blobbing or excess solder raised on your parts.

A leather blade washer cut so the shoulders of the blade put pressure against the leather then peen the tang same as in first method described. The British 1796LC sword was made this way.
When you find old swords with a loose guard but the tang is peened tight you should see a gap between blade shoulder and guard where a leather washer once was.

I should have specified the type of sword I was thinking about. British infantry and others with backstrap, fishskin covered grip wrapped in wire and a forward ferrle. Difficult to get the peen tight with just two hands, seems to bounce back when peening.
It can be done with two, just easier to have things fitting properly and lined up.
Other swords as described are easier as Eric says.

this discussion is really interesting
as I 'm the happy owner of the sword of Peter , I post the photo of the pommel after finish

Oh...my bad. Sorry bout that. :D