Muscle activity monitoring or Electromyography (EMG) is useful in gait analysis, injury prevention, computer or robot interfaces and assisting patients with communication difficulties. However, EMG is typically invasive or obtrusive, expensive and difficult to use for untrained users. A possible solution is textile-based surface EMG (sEMG) integrated into clothing and used as a wearable device. This is, however, challenging due to (i) uncertainties in the electrical properties of conductive threads used to construct electrodes, (ii) imprecise fabrication technologies (e.g., embroidery, sewing), and (iii) a lack of standardisation in the choice of design variables. This paper, for the first time, provides a design guide for such sensors by performing a thorough examination of the effect of design variables on sEMG quality. Electrical characterisation and sEMG measurements are performed, considering the effects of manufacturing imprecision. Results show that the imprecisions in digital embroidery lead to a trade-off between low electrode resistance and high consistency. An optimum set of variables for this trade-off is identified and tested with sEMG during a variable force isometric grip exercise with n=6 participants and compared with traditional gel-based electrodes. Results show that thread-based electrodes provide a similar level of sensitivity to force variation as gel-based electrodes with about 90% correlation with expected linear behaviour.