You can either use rubber bungs, depending on shape of cold feed outlet or, use pushfit stop end on expansion pipe, hold finger over cold outlet, cut pipe and blank both ends, then reconnect after with either pushfit connector or compression fitting.
If you're a DIYer I wouldn't advise powerflushing a system yourself. The system has only ever been used with low pressure applying pressure via the powerflushing machine could cause more problems that it'll cure.
You have to isolate the expansion pipe, normally connecting the cold feed and expansion pipe together meaning the whole system is flushed including the fill and vent pipes. Fitting a isolation valve so that this section can be shut off when flushing other parts of the system.
Modifications should only be made if you thoroughly understand the set up of the system - as if incorrect could leave you with a dangerous heating system.
As you have had to ask the question how to do it - tells me you don't thoroughly undertsand the setup therefore it would be dangerous to attempt this yourself.