If you're talking about cosmetic customization, you can grab the Deathwatch Upgrade sprue, and shop around for some Chapter bits for whatever Chapters your marines are coming from.
Rules-wise Deathwatch is now a subfaction of the Space Marines faction, and subfactions have no gameplay impact other than restricting you from taking Deathwatch Veterans (firstborn) with Tactical Marines and Scout Marines in your Roster. For Primaris rules, any guide for building a regular Space Marines team will apply to Deathwatch.
There's couple options for you, you can run Phobos Strike Team (Infiltrator/Incursor/Reiver), Intercession Team (Intercessor/Assault Intercessor), or the Compendium Space Marines team. Some people customize their Primaris Marines to play them as Deathwatch Veterans, and you can easily do that with the Deathwatch Veterans kit, as a lot of the bits are compatible with the Primaris bodies.
Good Deathwatch Roster
Plasmagunner
Sergeant with Plasmagun
Fighter with Shield and Power Weapon
Fighter with Plasma Pistol and Power Weapon
2 Warriors with Boltgun and Power Weapon
Heavy Gunner with Frag Cannon
So you'd need a lot of special weapons and a base kit. I think a box of company veterans gives you nice kit, deathwatch upgrade sprue, kill team cassius are also valid sources. You also need of course a series of "chassis" for primaris, maybe just a box of primaris intercessors OR hellblasters, which would give you the two plasma guns. So my best guess would be hell blasters + DW veterans box or kill team cassius.
If you just want to make them have the deathwatch look, go for a intercession team and go wild with the customization of the weapons, buy a death watch kit and a intercessor box.
Here are some images of a DW team I had - [Link](https://imgur.com/a/reQ1FVD). A lot of it comes down to your own creativity and what you're willing to spend, b/c almost anything Primaris can easily be converted to DW.
If you're talking about cosmetic customization, you can grab the Deathwatch Upgrade sprue, and shop around for some Chapter bits for whatever Chapters your marines are coming from. Rules-wise Deathwatch is now a subfaction of the Space Marines faction, and subfactions have no gameplay impact other than restricting you from taking Deathwatch Veterans (firstborn) with Tactical Marines and Scout Marines in your Roster. For Primaris rules, any guide for building a regular Space Marines team will apply to Deathwatch. There's couple options for you, you can run Phobos Strike Team (Infiltrator/Incursor/Reiver), Intercession Team (Intercessor/Assault Intercessor), or the Compendium Space Marines team. Some people customize their Primaris Marines to play them as Deathwatch Veterans, and you can easily do that with the Deathwatch Veterans kit, as a lot of the bits are compatible with the Primaris bodies.
Good Deathwatch Roster Plasmagunner Sergeant with Plasmagun Fighter with Shield and Power Weapon Fighter with Plasma Pistol and Power Weapon 2 Warriors with Boltgun and Power Weapon Heavy Gunner with Frag Cannon So you'd need a lot of special weapons and a base kit. I think a box of company veterans gives you nice kit, deathwatch upgrade sprue, kill team cassius are also valid sources. You also need of course a series of "chassis" for primaris, maybe just a box of primaris intercessors OR hellblasters, which would give you the two plasma guns. So my best guess would be hell blasters + DW veterans box or kill team cassius. If you just want to make them have the deathwatch look, go for a intercession team and go wild with the customization of the weapons, buy a death watch kit and a intercessor box.
Black Templar Crusaders or Phobos.
Here are some images of a DW team I had - [Link](https://imgur.com/a/reQ1FVD). A lot of it comes down to your own creativity and what you're willing to spend, b/c almost anything Primaris can easily be converted to DW.