How to make houndstooth pattern seamless/tileable?

You can use Object→ Repeat>Grid and then locate them as you want. Then you can create pattern.

​@CORCTON34033231 You need a much larger tile than this single object to be able to repeat it as a seamless pattern. The easiest way to create the pattern is to make a horizontal repeat tile first.

You can always rotate the pattern when you use it to any other angle.

I’m not an expert in pattern design, but from what I see online houndstooth pattern looks differently. The pattern you want, can be achieved by copying, moving and rotating with transform effects. 

But is this really the pattern you want to have or do you want to have what is considered houndstooth pattern by all the others?

Hi! The easiest way is to turn your base shape into a Pattern swatch, instead of copying it by hand:

1. Select your base shape.

2. Go to Object > Pattern > Make.

3. In Pattern Options, keep Tile Type as Grid, and turn on Show Tile Edge if you want to check the tile.

4. Click Done — the pattern is added to the Swatches panel.

5. Draw a shape and click that pattern swatch to fill it; it will repeat as a seamless tile.

If you still see a gap or a cut at the edge, select the pattern in the Swatches panel to edit it, then adjust Width, Height, H Spacing and V Spacing in Pattern Options until the tiles meet cleanly, then click Done again.

Official guide: https://helpx.adobe.com/illustrator/desktop/paint-and-fill/create-and-edit-patterns/create-patterns.html