Hi Gerry,
The answer to your question depends on the signal strength in your area.
If it is strong where you want to split your signal (after the booster) then there should be no problems with splitting the signal passively. If not, then you will need an active splitter (as you already have one with spare outputs, then that is when you need to run a cable to your loft).
As you state that you are coming out of a booster, then you should be fine to split that passively. The easiest way to do this is by buying a splitter with F-connectors on it and a pack of F-connectors. These just screw onto the cable and then into the splitter.
It may be tempting (read: cheap) to use a choc-blok or similar to make your cable. Please don't do this, as it will degrade your signal substantially.
As always, if you are going to do this by running a cable outside, use outside rated cable and an outside rated enclosure with suitable grommets for your splitter.
HTH,
Matt