well i have this question guys, how do they does this? every ship would need a navigator?, can they jump through the warp and just follow the ship where the navigator is?, how do you handle this?

Any ship that's not doing tiny jumps of a few light years in the warp really needs a few navigators aboard.

Travelling as a fleet is possible from the fluff but tricky to coordinate - handle it as modifiers to the navigation warp tests based on how many ships are trying to travel together.

If you run with one ship having navigators and the rest follow they would still occasionally get separated (warp storms would do it) - in that case the ships without should drop out of the warp and then use astropaths to try to contact the 'mother' ship to come pick them up.

All of that takes time so perhaps double the journey time for fleets (and thus twice as many warp encounters).

It can get messy. The fluff has examples of fleets, or even big ships, and their escorts, jumping, only to have some element come up missing. The Light of Terra's escort fleet was very surprised when they appeared, and the battleship was no longer with them, while the opposite, a battleship appearing in a war zone, without her escort, would suck, too. I like to think this is one element that is meant to stop every Rogue Trader from flying with a small fleet; if you have a cruiser, you really should have a frigate, or maybe a small group of raider-class vessels following along, making up for your slow response times, but this can get expensive, even in the arena of those who can afford to upkeep multiple ships, in a single endeavor. If you have to pay for a couple Navigators on each ship, you quickly stop making as impressive of profits, and this gets you to go it alone, so that your Storyteller doesn't ALWAYS need to plan ship encounters around you having five ships.

If you really want to, you could chain the ships together, and strain your warp engines, and Gellar fields, to cover the whole group, and tug them all through the warp; this is how they seem to describe space stations, like the Ramilles, being moved, but it is sort of silly, and very dangerous. Otherwise, I'd say you can have all the ships make a check, but then use the worst result to cover them all. If not, each needs to check, and go with what happens to each of them. Some ships are slower, and even fellow Navigators see the Warp, and the Astroomicon, in different ways, and at different qualities. Lastly, if you like ripping off the Tau, you MIGHT persuade your Storyteller to allow you to build a clamp, if you will, onto the biggest ship, to carry rather smaller ones. This is how the Tau battleships carry their escorts, though they do use the Warp differently than the rest.

In one of the BL novels about the Damocles Gulf Crusade, even though they all arrive at about the same time, the RT fleet is scattered around the edge of the star's gravity well and takes some time to regroup.