How and When to Send?
When you send your save the dates out depends on if you are having a destination wedding or not. If guests need to book flights then sending your save the dates out up to a year ahead is the best plan of action. If you are just giving your guests a "hey don't arrange anything else on this date", then you can leave less of a gap.
Should you make the save the dates yourself? Printing your own save the dates is fine for a less formal wedding, but a formal wedding should have professionally printed save the dates, although of course you can still design them yourself. A formal wedding is also likely to use a standard save the date template, as opposed to a unique save the date idea like using a luggage tag.
Adding a signature to the end of the save the date, or a personal message on the back is acceptable if you are having an informal wedding, and using address labels is acceptable whether you are having a formal or an informal wedding.
Depending on the style of your save the date, you could either mail it out as is - postcard style - or you can insert it in an envelope. Either is fine, and is also dependent on other details or inserts you are mailing your card out with. Usually the formal approach would be to use an envelope.
Do make it clear who is being invited. Of course this may change due to separation and so forth, but you need to make sure the recipient knows that their partner is invited too, otherwise they can't really make suitable arrangements. It is perfectly acceptable save the date etiquette to include "and guest" on the envelope.