There are two ways of doing this; they are explained in the latest edition of Mass of Ages. In case you've not got your copy (no?), here a summary.
One is called 'easyfundraising'. If you visit your usual online retailers through the easyfundraising site, or using the easyfundraising app, having told easyfundraising which charity you wish to support, a great many of them will make a small donation to your charity if you buy something.
Just visit the easyfundraising site and all will be explained. The donations vary in size; some are fixed sums, some are percentages. It's not much but it adds up. If thousands of UK Catholics consistently did their shopping this way in favour of the Latin Mass Society or Aid to the Church in Need, it would raise a very real sum of money.