IG send you an email asking you whether you want to participate...

IG send you an email asking you whether you want to participate in a SPP with a reply by date.
Apparently all you have to do is reply to the email stating how much you want to apply for & they take the funds from your account.

Assuming there are no issues with the emails the only outstanding question is how long IG take to get the shares in your account after they are issued. They'd have to be pretty poor to be as bad as IB have been on some of the recent SPPs.

Only time will tell & unfortunately we have no idea when or what company will do a SPP.
There was a big glut pre Christmas so I'd expect it to be a while until the next one.