Ade,
If you are using Tenacity 5, then you will have very little trouble.
Mix it up with a few drops of water and the tiniest drop of washing up liquid, after getting it to a mixture like pouring cream, paint it on to and into the areas to be joined. Don't go mad, the more you put on, the more you have to get off. Just use a thin layer on each.
Assemble the two parts together and stand it up on your firebricks.
Now form a ring of silver solder that is the same size as the circular joint, and place it on top of the joint, onto the flux gathered around the joint.
Now heat up the job from the outside, starting roughly where the depth of the joint finishes and upwards. When everything gets to melt temperature, the silver solder will melt and by caplliary action will form the joint. DO NOT point the flame at the silver solder, only onto the outside of the job. Unless all the silver solder you have put onto the joint is wicked in, you shouldn't need to feed any in from a rod. If the joint still requires a bit more solder, then still keeping the job hot by heating around the outside, feed in a bit more rod until the joint is full. The solder should start to flow when the parent metal reaches a very dull red colour, any hotter, and you are liable to cook things a little too much.
Be very careful when you first start to heat the job up. Because the two parts resemble a piston in a cylinder, the evaporating water in the joint might try to push the inner part out. Just hold it down with a bit of metal rod until all the steam has gone from the joint.
A lot of people struggle with silver soldering, but if you have the correct solder, flux and heat in the right place, then it is in fact difficult not to form a good joint.
There are lots of my posts showing how I do it.
John