Hi, I took so long writing this someone else best me to it !
If it will fit turn the axle round so the cone and nut that is already tightened goes in the freewheel side, then tighten up the side you can get at.
Has the washer on there got any flats on the inside to match the axle ? Just wondered if it was a locking washer like you have on old headsets. If it is then you do need to fit it between the locknut and the cone.
The rest is as the others have described.
What I used to do was fit the axle with the locked cone and nut on the freewheel side, then fit the other side. Tighten the cone up finger tight and then fit the locknut. You need to hold the cone and the locknut with spanners and tighten the locknut against the cone. This normally makes the hub a little loose as the axle twists a bit as you tighten the locknut. What I do then is losen it off again, re-tighten the cone finger tight and tighten slightly more with a spanner (only a bit though or you might damage the bearings) and then re-tighten the locknut. You might need to do this a few times until its right.
The only thing I can think could be a problem is that on a bike I used to have you couildn't get the cone out of the hub with the freewheel on but I don't think yous is like that.