The Downing IFISA is a new account that you can open on www.downingcrowd.co.uk in addition to your standard account.
Once opened, you will need to make a cash deposit (either as a new subscription or by transferring across an existing ISA) before you can invest in a Crowd Bond through your Downing IFISA.
The annual subscription limit of £20,000 in the 2019/20 tax year relates to the money that you have deposited into your IFISA, not to the amount you have invested - so even if you open an account late in the financial year, it doesn’t mean you have to immediately invest it all.