Garage Wall advice

[matt173407]

Any advice would be appreciated as I am clueless

as image shows have a similar garage with the old concrete panel walls, currently converting to man cave ideally I would like to cheaply and easily insulate walls , I have been advised on celotex as an option, any advice on what a hopeless DIY person can do as a simple option, in ideal world would like to stick whatever solution to the walls rather than drill any advice or mockery appreciated

[MDMA .]

you could batten it out and insulate between the battens before plaster boarding the tops. TBH, the majority of the heat loss will be through the roof. what is it at the moment ?