I've found it near impossible on my own without a tool like this. You could make something up a bit simpler maybe. This one was one of a small batch made by another forum member. Ron
I made mine (hopefully see picture above). You attach some stiff wire to one end of the spring and wire it to the loop. Pull the spring on and then snip the wire.
Takes a minute to do.
You need the panniers off the bike if they are fitted.
An alternative to Pat's idea is to use a suitable length of stout timber and put the wire around it. A pick helve or sledge hammer shaft is fine. Rest it against the back tyre and lever away. If you have someone who can ensure it is pushed on to the 'pip' then it is easier still. But it is a feasible one man job.
email (option): dncnsimpsonAT the well known 'warm'mail.com