Build it as two armies standing near each other, because the detachment rule pays for exactly that.
The Guard half. Check your shopping list against the exclusion list before you buy anything, because Faction Pack v1.1 rewrote it and it removes several units a Guard player reaches for first: no AIRCRAFT, COMMISSAR, EPIC HERO, MILITARUM TEMPESTUS, OGRYN, RATLING, TECH-PRIEST ENGINSEER or MINISTORUM PRIEST. A Genestealer Cults model must be your Warlord, and your Astra Militarum models lose Voice of Command, so do not build around Orders. Bring at least one Astra Militarum vehicle, because In the Shadow of Iron names ASTRA MILITARUM VEHICLE specifically and turns a tank into a mobile Cult Ambush anchor.
The Cult half. Purestrain Genestealer broods, and a Patriarch if you want Talons of the Sire at 15 points. Mark of the Star Children at 30 points is the bigger investment and changes how a brood dies: +1 Toughness, a 4+ save and +1 Strength in melee is not the model your opponent has planned for.
Enhancements to prioritise. From Brood Brothers Auxilia, Martial Espionage (25 pts) improves a Guard unit's ranged Armour Penetration by 1 once per turn and is the most direct firepower upgrade available; The Hero Returned (20 pts) adds Leadership and Objective Control to a Cult infantry unit, which matters because a battle-shocked unit has Objective Control 0. All four of that detachment's enhancements are GENESTEALER CULTS INFANTRY model only, so none of them go on your Guard.
Stratagem to remember. Symbiotic Destruction for 1 CP is the pairing in miniature: one Guard unit and one Cult unit that have not shot both lock onto the same target and both re-roll Wound rolls of 1 against it.
What we cannot sell you. Our catalog holds no Genestealer Cults single-unit box, so none of the Cult models above carry a box link. [Combat Patrol: Genestealer Cults](/40k/boxes/combat-patrol-genestealer-cults) is the entry purchase; check its box page for current contents and price. And note that [Kill Team: Brood Brothers](/40k/boxes/kill-team-brood-brothers), despite the name matching this detachment, is a Kill Team box rather than a source of the Astra Militarum datasheets this detachment actually imports.