Dumb question re wheel arch tubbing

Folks,

I haven't done body work for over 30 years..that is why we bought a rust free duster however, to get any decent sized tire under the rear, I am now thinking we will need to tub out the rear.

Could anyone post a step by step guide..I suspect there are a few other newbies that would appreciate it.

I forgot to ask..are there any downsides to big rear tires ?

Thanks
Ian