You chose your aim / risk profile - which is a choice of 3, pay your money in and that is it. They manage the portfolio and investments and you see what happens. Literally nothing to do
For the investing side, Stocks/Shares ISA. A number of websites will offer ready made packages with different risk levels depending on your aim. As these are within an ISA wrapper they are tax-free.
I used The Share Centre who have a great website, easy to use and a Cautious / Positive /...