Awright, so this is what you meant - german armor is not my speciality.
Looks like a nice thing to do, too - you basically have to add some material to be able to cut and sand later. The material you want is styrene - the same stuff the kit is made of. First choice would be to take some unused part from the same kit or from another kit. You can also buy sheet styrene on the net and some people even buy various signs to cut them up and use for material - you just have to watch out because some of the signs are different material than styrene. Or you can take a close look at what you throw out and use that for free - junk with PS in a triangle is styrene and can be cut up and used in models. For example where I live big ice cream boxes give me lots of thin styrene sheet.
Once you have the material to add you need to cut it up (tools to do it could also be described in length) and glue it - here again I recommend lacquer thinner for a very strong bond - it just takes some time to cure. But the bond is so great you can glue two plates that are coloured differently and if you drill a hole in that you get a two-coloured burr - this should give you some idea.
Cutting and filing the desired shape then almost seems trivial! Welcome to the fantastic world of scarchbuilding! Good luck with it and have a nice day
Paweł
PS. Please don't forget to post work-in-progress (WIP) pictures