Make the chain line as long as possible (usually by releasing all air from rear shock and then using some bungy cords or rope to keep the bike compressed)

then I make the chain as short as possible by running the chain through the biggest ring on the back through the rear mech, looking at the position of the rear derailer when the bike is fully compressed this will stop you making the chain too short and firing your mech into you rear wheel.

this is how you derailer should look with the suspension fully compressed. I have found this method to work really well and have had no issues.

chain length Article.

takisawa’s trick works on most bikes but might end up with more chain than you need- I do much the same as passtherizla, put it in bottom gear and cycle it through the travel (remember the chain isn’t neccesarily longest when the suspension’s fully compressed)