There's no 'best way', everything depends on what you want.
If you want to buy something which, like a brand new car, loses value as soon as you take ownership and will never recover that value, then sign up for a standing order with Royal Mail. You - and an estimated 30,000 others - will get every* new issue on a Royal Mail fdc with your name and address ink-jet on. The postmark choice is a pictorial Tallents House or pre-determined alternative.
In 5 or 10 years - or 50 years - time if you or your heirs come to sell them you or they will find that few people want RM FDCs from their standing order service. Such FDCs issued in the 1970s-1980s can be bought from bulk dealers for as little as 25p each (compare that with the face values) - so you can imagine that the sellers don't receive much.
For every stamp issue there are other postmarks:
If you want one of the other postmarks, and an unaddressed cover, which will better hold its value, there are other options, from DIY to using a local dealer or specialist FDC producer. But be prepared to pay more for this service, and still collect because you like them, rather than to be certain of a monetary return.
Read the advice on this page http://www.buckinghamcovers.com/covers/index.php
- there is a great deal of good advice there and although Tony Buckingham has a website to sell the covers they produce, he also does his best to steer you away from the covers that are less worthwhile.
* Caveats over what you will get from Royal Mail.The Diamond Jubilee stamp
is a good example of RM's limited capacity for serving the collector. The official FDCs for that issue had the miniature sheet, which contained 6 gummed stamps. I produced FDCs with all four 1st class Machin on: the one from the miniature sheet, and the three self-adhesive stamps from the counter sheet, business sheet and booklet. These are all separate stamps, all with individual catalogue status, but ignored by RM for FDC purposes.
For prestige stamp books, the RM fdc only ever has the block of Machin definitives (without the narrative part of the booklet pane) and they ignore the rest of the booklet even if there are new stamps. Serious collectors of PSB FDCs have all 4 or 5 full panes on individual fdcs.
Of course if you have time and a decent printer you could do it yourself, to create something unique (or almost).
Philatelics GB stamps info: http://www.norphil.co.uk
- also /catalog for our ecommerce site, blog.norphil.co.uk, shop.norphil.co.uk and Ian_norvic on twitter