^ Agreed. Spirits can usually let you know, through intuition or symbols if not outright conversation, if there’s a definitely-right or definitely-wrong time to call them, so if the time you have in mind feels right, then trust that.
The sigil will become consecrated by opening it, but you could first bless the paper and pen you use, either in general (if you compile a box with paper and pens for this work) or for this specific ritual. If you “give” a pen to certain forces, don’t use it for other work with different forces or spirits later, as a broad rule of thumb.
The simplest method is to take your right forefinger (unless left-handed, in which case, swap) and first twirl it 3 times over the item counter-clockwise, saying something like “I cast out any impure and adverse energies from this pen/paper/whatever” then flick your hand towards the window, door, or any burning flame (not the candles you’re going to use) - as a kid, I used the bars in the electric heater for this. Imagine someone scooping cotton candy on a stick, then flicking it away to be dispersed.
The do the same clockwise, “I bless this pen/paper/whatever for the purposes of (state the point of the work and name of any spirits) and command it may remain pure and perfected.”
With variations (“pure” isn’t a concept some spirit like), I’ve been using this method for over 20 years, and spirits seem to find it acceptable - and you can often feel the energy change in the blessed item afterwards.
You can also use the “casting out” phrased respectfully to remove the link between a spirit and their sigil after use if you’re going to destroy it - I always do this, for example “I release all energies of N. Spirit from this sigil, may it become once again a mere thing of paper, and all links and bonds dissolve.” Belt and braces maybe, just sharing some personal lore. 
There are established wiccan methods of using all the elements or whatever to consecrate items, and maybe you have your own, this is just what I like to do to bless stuff.