Frog construction question

[FJ and G]

You guys who lay track and make your own frog, this Q is for you. I want to fill the gap with solder (O scale) and then cut the flangeway with a hacksaw blade.

Some questions:

Do you build the frog first and then set it in place or do you lay the rails to shape the frog and then fill in the gap?

If the latter method, do you fill the gap with solder and if so, do you set a thin sheet of brass so the solder can build up?

Also, is this brass placed under the ties or over the ties and if over the ties, how do you get the correct measurement of the sheet of brass?

Thanks.